>>17047257HG/SS was because, while they were OBTAINABLE, there were a few that had rather convoluted methods involved, and there were a few issues with the main gen 4 games that HG/SS handled better.
Also, Gen 2 had the distinction of having the longest main quest in the game, and by remaking it they effectively "remake" Gen 1 as well, since Gen 2 featured Kanto.
In addition, the HG/SS games were the way of getting Gen 3 starters in the DS games.
Yes, that's just as bizarre as it sounds. A lot of people tossing around da mudkipz right now would probably be able to trace its lineage back to HG/SS rather than Gen 3.
