Just to confirm, this might be an opinion-related topic, and I don't think they are allowed here. I'm not sure if it's considered that though.
Anyhow, I think they deleted it because the next games (Black and White) would unveil more Pokemon than any other generation Pokemon. I don't think they wan't to create another 160 overworld Pokemon sprites.
But yes, I enjoyed that feature very well. But I don't think we'll be seeing it any time soon, with just over 700 Pokemon I don't think they'll be interested in doing that feature again. :(