5th Gen How did you think they got the idea for BW1 to only include new creatures?

What the title says.

Honestly, to make it a bit of a reboot and keep things fresh (this is my guess). Remember that something like half of Gen 4's pokédex were pre-volutions or evolutions of existing pokémon. It made Sinnoh feel like a big, well established area near Johto and Kanto that wasn't too alien. With Unova, their whole deal was a region far far away that is very different with all different pokémon and a different culture. A completely different pokédex let them create a completely new unique tone and really make it fresh.

But we sure did miss our old favs. People have said that part of why SwoSh is so strange is that many old favorite pokémon aren't accessible anymore. We had to play through BW's story without any of our old favs accessible. It probably irked many. But yeah, my thoughts are here above :3

^Yeah, basically this! I remember some people were really unsure about the fact that we'd be limited to new mons only (same with me at first), but I started enjoying it not long after and now look back on it fondly. The temptation to use the same old Pokémon is pretty high sometimes when I've used them before and know they can do well haha, so being forced to play totally new is refreshing. I honestly hope they do something like that for a new gen again!

They probably decided "we want to (soft) reboot the franchise!" Which is something Pokemon always seems to do every odd numbered generation. Generation 3 was sort of a reboot, as well. Old Pokemon weren't in the first few routes of the games. But, it's definitely because Game Freak wanted to reboot the franchise again. Also, Unova is extremely far away from other regions while Kanto, Johto, Hoenn and Sinnoh are likely not too far away from eachother. That was probably done deliberately, though.