Verified answer

They decided that it was enough to remake Red and Blue and then Gold and Silver. Yes Yellow was awesome because you could get all the Kanto starters and Pikachu followed you around, but all pokemon follow you around in Heartgold/Soulsilver and trading is so much easier than it used to be. I think a decent part of it might also be that too many people would complain that they're just rehashing too much of the old stuff rather than giving us new games (even though I still prefer the pokemon from gen 3 and before.) Sorry.

As stated, Heart Gold / Soul Silver did feature the best part of Pokemon Yellow - walking with Pokemon.

They also followed the main feature that made Crystal different from Gold/Silver- in that Suicune was present, and coincidentally happened to be in several places you needed to go when you were going. (Pokemon was pre-emptively following us or something, though I think we were supposed to be following it. But really, we just seemed to end up in the same place at the same time a lot.)

So they probably didn't see a need to make another game that was essentially Heart Gold / Soul Silver.

Besides, they were also making the new games as well, I'd imagine. They can't just devote all their time to remaking entire generations of older games. Especially when they can combine the features of the older sets into just two games per remake.