So is there any reason why the ladder is so different from what the viability rankings would suggest? Is it just lots of poor players, some serious people particularly enjoying one archetype, or what?
There are several different reasons for the phenomenon you have observed. First, yes, the ladder has a number of poor players, in addition to some good ones. There are a lot of literal kids just having fun, who never read the VR, and there are people who primarily play in OU or other metas just trying out doubles with mons they like. There are a number of gimmicks that are only viable in doubles. Beyond that, there are some mons that may be more interesting/fun to play than others. Among the ones you mentioned, Regieleki is ridiculously fast and powerful, Whimsicott has prankster and beat up, Tyranitar is a weather setter (especially important in doubles) and Dracovish is super powerful when it outspeeds. Very good players can play around these mons better than your average ladder denizen, making them less attractive for the VR. Spectrier only has one strong attacking move and can feel one dimensional, though it's very good when played right. However, when constructing teams, you also also have to look at possible substitutes. Spectrier can hit psychic and ghost types super effectively. Well, Incin can do that and so much more. So even most very good players end up picking Incin over Spectrier for their balanced team.
If you're just looking for a way to be good on the ladder, the VR is absolutely the best resource out there to get you started. However, you should understand that it was built by people who have just about 0 respect for the ladder with just about 0 input from ladder usage (not sure if the disconnect is as complete in other metas). It's built to reflect the current tour bros metagame (mostly comprised of say 50 or so experienced tour bros playing each other and chatting with each other about it). Therefore, it evolves significantly over time as the tour bros meta does. The ladder may evolve in different ways. Tour bros and ladder heroes both play to win, but they have fundamentally different objectives. To be successful in tours, you have to beat very very good players in the tour bros meta more often than not and gimmicks don't really work (because most tour bros have seen them, and you are probably going to reuse your team, such that even a gimmick that worked once probably wouldn't work again). To be successful on the ladder, you need to beat all kinds of different teams with more varied mons, usually ones lower ranked than you, very consistently. The best ladder heroes play a LOT of games and have like ~3-4 to 1 win loss ratios (even after getting to the highest level). If a tour bro ever tells you that there's 0 difference between what's viable in tour bro tours and what is viable on the ladder, they are not a top ladder player.
If you want to invest a lot of time in getting good at the DOU ladder, you can definitely start with VR, but also watch the top players on the ladder. Watch their matches and replays. That's ultimately what you want to be, so you can learn from them. Use what works for you on the ladder. There are definitely a few unranked mon that you could make work on a ladder team after you get more exposure to them, but you really can't go wrong by starting out with a balanced highly ranked VR team like the one you have (though you'll have to figure out how to deal with all the room/semiroom teams out there).
As for dnagerbadger saying don't try to help people on the ladder? The affectation of tour bro elitism from someone like him is pretty silly (and it won't even help him get drafted for their tours). There are people on the ladder who just want to have fun and many others who genuinely want to learn and appreciate advice. Strike up a friendly convo and you can find out ("nice team" if the opposing team really looks good is often a good starter).