Because as people migrate to the new games, there's no point to continuing to play the old ones? Particularly as that migration often involves moving Pokémon ahead, a one-way journey that leaves one's older games barren of anything worth battling with.
This pretty much hits the nail on the head. Speaking from personal experience, every Pokemon of worth that I've ever caught has been moved to Gen V, since it's the most current and up to date. Nowadays, the only thing I use past gen games for are special runs (Nuzlocke, Monotype, etc.), or if I need another Legendary from that version. But when the bulk of the battling/tournaments are done exclusively for 5th Gen, as are the giveaway events, there's little point in investing too much time into Gen IV or anything prior.
I make no claim of being perfect or always right. Take everything I say with a grain of salt.
If you're ever feeling sad, do a Pokemon fusion with Weepinbell + Anything.