There's definitely an interesting part to teambuilding that is worth bringing up. Namely, I feel a lot of teams in a given region are driven not just by the availability of Pokemon, especially new vs old, but also what's particularly good. Something that's very useful, is fun to use, and tends to look cool or cute design wise in a memorable way. And is readily available in some way.
Sinnoh gets memed a lot for having a very specific team composition but there's a good reason the same set of its new mons are so commonly used. Luxray and Staraptor are presented extremely early on as the relatively cute Shinx and Starly, making them appealing off the bat when you're just starting out and catching new Pokemon, and they evolve twice over the game into middle forms and then really cool and badass final forms, with strong overall stats and good toolkits to make them valuable team members. They are good early-game Pokemon in just about every front, which is very rare when most early game Pokemon are a combination of mediocre and bland looking and will inevitably fall off. When you consider that is the norm, having something immediately appealing that also lives up to that appeal off the bat makes Starly and Shinx quite standout. Bibarel also gets on so many teams because it has a funny design but most of all even if people don't use it in combat, it learns so many HMs in an HM heavy game and is available everywhere in the region, so it's easy to pick up and throw HMs on it for field use, also making it useful as a field member.
And then you get Budew, Buizel, and Ponyta fairly early into the game, and they are three elemental users who are immediately valuable for covering your starter in type matchups, with Budew also going through a great three-stage progression into Roselia and then Roserade. Other cool Pokemon are presented to you in appealing ways as well. Riolu is presented as an egg that you get as a gift in the story and becomes Lucario who is one of the most marketed and cool Pokemon. Garchomp is found as a Gible in a side dungeon and is not only really awesome looking and powerful, but is one of the few pseudo-legendaries who is genuinely usable in an in-game playthrough thanks to a good natural movepool and reasonable evolution levels. Weavile and Gliscor (in Platinum for the latter) are cool new evolutions who have great designs and their pre-evolutions are found in distinct places, and despite Weavile, Garchomp, and Lucario being available fairly late, make it to their final forms at a reasonable time to justify their use anyhow.
Meanwhile addressing Kalos since that was brought up. It seems to be on the other end of the spectrum. Talonflame is popular because it's also a really good and cool looking early route bird like Staraptor is, but then you see almost everyone use the Kanto starter and Lucario, because the game railroads you into them by giving them to you as gifts. They are a case of gift Pokemon that are given to you, but you're inclined to keep them because they're just so good. The Kanto starters and Lucario are all popular and have great designs, and giving them Megas to highlight the new feature with new designs for all four of them no less gives them novelty value despite being old gen Pokemon, and using their Megas feels good, especially since the former are basically your second starter and now you get to Mega Evolve them, while Lucario is the ultimate reward to use Mega Evolution, which X and Y slowly builds up.
And then the last two slots are usually some sort of fill-ins. Lapras is sometimes used, though not always, but it's useful for Surf off the bat if you need it and has a special overworld sprite to top it off. But usually I see at least two of the following: Sylveon, Aegislash, Goodra, Heliolisk, or Noivern. Kalos does have a massive dex with a small new dex with most of the new mons scattered across one route apiece. That said, each of the new Pokemon only being available on one or two routes does work to their benefit in some ways: while it has a small new roster, many of the new Pokemon are good, with standout designs and great battle capabilities, like Sylveon being the new Eeveelution and the poster child of the Fairy-type, Aegislash being a unique gimmick Pokemon with a great type and skillset, Goodra being a usable pseudo-legendary, and then some other particulary solid picks.
What I do think is the benefit of the new Kalos mons often being only in one route and often intermingled with many other old gen Pokemon in a large regional dex is that it helps the new Gen 6 Pokemon stand out more, and it feels good when you do find them and use them, which I think has allowed some of the really like Goodra and Aegislash to be desirable and standout picks.
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That said looking at BW1 in particular, it ultimately has a few shortcomings in how its new roster is designed, especially using the above notable aspects of Kalos and Sinnoh as comparison. The first two routes before the first Gym have a dismal selection. It's just Patrat, Lillipup, and Purrloin. Patrat is mediocre, Lillipup is fine but nothing worth noting, and Purrloin stinks. Watchog and Liepard aren't exactly great either. Lillipup is the only one of the three who has any sort of longevity. Contrast that with DP's immediate giving of Starly and Shinx, and even mingling in a few other good early mons before the first Gym like Budew, Geodude, Psyduck, and whatnot as other potentially good options. Then there's your first "gift" Pokemon...in the elemental monkey. The monkeys are not horrible in battle per se, but they're pretty weak, don't have appealing designs, and even when they evolve their evolved forms are widely considered ugly and battle wise they're quite bland and average at best and later on it shows. Whereas XY gives you the Kanto starters and Lucario who are really good and showcase a new and cool mechanic, BW gives you a boring monkey who only serves to help against the first Gym in a tutorial on type matchups and is a very bland Pokemon otherwise in just about every aspect.
Most of BW1's early roster consists of Pokemon who are functionally identical to something from an earlier generation, and while Gen 5 does have some really cool and fun Pokemon, many of them are either found very late and evolve way too late to be of use for a playthrough or are fortunate to be in the mid-game. From my experience, there are precisely two points in the game where fun team picks are presented to you: the desert around Nimbasa, and Chargestone Cave. The desert has some really good ones, like Sandile, Darumaka, Scraggy, and the two fossils, and the area around Chargestone has some great ones in Joltik, Ferroseed, Tynamo, and to an extent Klink. Litwick and Axew can be found around the same point in the game through other means. But those are generally the main points where something interesting and good is available.
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I do want to say though, I think SM was somewhat worse with mingling its new Pokemon into the region than XY. Mainly because so many of them are slow or boring, and are mingled in with what is often time a repetitive combination of Pelipper and Yungoos/Alolan Raticate. Everywhere you go, it's Yungoos, Alolan Raticate, and Pelipper. All. The. Damn. Time. And then you find very few good Pokemon here and there, but I feel a lot of them aren't too standout or fun.
Just a lot of rambly thoughts on some topics here.