Keep in mind in Gen 3 there weren't a lot of Gen 1 Pokemon running about.
In Gen 4 there weren't a lot of Gen 2 Pokemon running about.
Majority of the Gen 2 ones that appeared in Gen 4 were related to Gen 1 or Gen 4 Pokemon, but still they kept out a large number of them from the games until the remakes were made, this includes Legendaries.
This isn't true in B2W2. Pre-Game the Pokedez has around 32% of the Hoenn Pokemon in it and Post game I think it rises a little, since we don't have a full list I can't tell exactly how many are in but from Pokedex and Swarm alone its about 40%, this doesn't include interact-able and Pokemon that are just added to the routes and caves/Forests. The number could go up to 50% at least with all the info I'm missing.
This broke the chain for this combo, this is why I don't think we'll be getting Gen 3 remakes until Gen 6 cause that one has a high chance of having less Gen 3 Pokemon and fits the platform pattern.