I definitely fall on the side of thinking that Kanto Pokémon are favoured because they're popular, not because they're from Kanto, and that the fact that the most popular Pokémon are from Kanto is only coincidental. It's a self-feeding cycle to an extent with characters like Pikachu, Charizard, Gengar, and Lucario, but as my inclusion of Lucario demonstrates I don't think that this is unique to Gen I. Pokémon from later generations I would identify as being in the same feedback loop are (Gen II) Togepi, Pichu, Umbreon, the Legendary Beasts, Tyranitar, Lugia, (Gen III) Grovyle, Sceptile, Torchic, Blaziken, Mudkip, Swampert, Gardevoir, Aron, Aggron, Manectric, Milotic, Absol, Salamence, Metagross, Rayquaza, (Gen IV) Turtwig, Infernape, Staraptor, Garchomp, Lucario, Munchlax, Rotom, (Gen V) Excadrill, Whimsicott, Darmanitan, Chandelure, (Gen VI) Greninja, (Gen VII) Rowlet, Incineroar, Rockruff, Lycanroc, and Mimikyu. I think it's too early to tell for Gen VIII, though Wooloo is definitely a contender to be on this list.
These Pokémon are listed because I think that, for whatever reason, their popularity has endured better than other Pokémon who TPCi have attempted to put into their own feedback loops. After Pachirisu won VGC they actually increased their production of Pachirisu plushies expecting an increase in demand, but it never came. Braixen and Weavile, two great designs with every chance of becoming all-time favourites, were included in Pokken Tournament but increased popularity didn't follow. Pumpkaboo and Gourgeist were pushed hard in Halloween season during Gen VI, but they never became popular either. Delibird is another case of a Pokémon who's still pushed every Christmas but who never has enduring popularity at any time of the year besides that.
The short of it is that though the trend above demonstrates a general decrease per generation, the outliers being Gen III with many enduring designs and Gen VI with practically none, that's looking at the state of things as they are right now. There are countless Pokémon from the more recent generations who TPCi has tried to make as popular as Charizard or Mudkip but they just haven't risen to the occasion and actually found the popularity that's needed for TPCi to keep pushing them. This is the same reason why despite being Kanto Pokémon, we don't see Dewgong or Poliwrath merch anywhere, because they're not popular enough to warrant it despite being in the series from the start. Most designs which TPCi identifies as having icon potential are tested to see if the general public is receptive to them, and we have slam dunks in more recent generations like Mimikyu, Lycanroc, Chandelure, Greninja, and Lucario to prove that it's not that TPCi is just sticking to their laurels by which I mean old favourites, because if they were these Pokémon would have never taken off to the extent that they have.
However, there is an issue, and it's that with each passing generation there are more Pokémon trying to share the limelight and therefore each one gets less of it. Frosmoth couldn't get a beautiful iridescent G-Max because they had to make one for Pikachu, Eevee and Meowth. Palossand didn't get a regional Galarian form (imagine one based on the White Cliffs of Dover!) because it was Gen V's turn to get some makeovers rather than Gen VII's. The more assets they have to balance the harder it becomes, and so necessarily the weaker or more nichely popular designs of newer generations can't compete with old Pokémon who were nichely popular but built up a fanbase over time. I feel confident in saying that Hatterene is a better Gardevoir, Shiinotic is a better Amoonguss, and Clauncher is a better Krabby. But Gardevoir was Wally's ace, Amoonguss has been holding down the fort in VGC since its introduction, and Krabby had that scene at the beginning of PMD Explorers where it blew the beautiful bubbles over the sunset. They've been in the series for longer so we have more memories with them, and therefore we feel more of a bond.
I think it's difficult to gauge the bias without looking at time frames. It's been 8 years since Gen VI and 24 since Gen I. That means we've been with Gen I designs for thrice as long as Gen VI -- many people who grew up with Red/Blue are married with kids of their own at this point. Perhaps in 16 more years we'll have a greater affinity toward certain Gen VI designs just due to their inclusion here or there, and we'll be arguing about Gen VI bias in the Gen XV games. It's unjust to expect new Pokémon to reach the levels of widespread popularity as the Gen I Pokémon because don't forget, the Gen I Pokémon are some of the most popular and known fictional characters in history. It was a social phenomena. If you were a kid in the 90s, your Grandma can tell you who Pikachu is, your parents can probably name a good 10-20 of the Kanto Pokémon, and half the people you went to school with could take a good stab at naming all 151 even now. In some ways, Pokémon is still using this early momentum, which means they have to be moderate in how they add to and revamp the series, meaning they can't stop favouring these early designs in favour of new ones. I mean, they tried that in Black/White when they only had new Pokémon until the postgame, and everyone hated it.