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It was starting to die out during the last year of gen 2, about 2001. The anime was bloated and dragging. The trading card game was getting repetitive and less interesting with the e series sets. (They printed so few because of low demand some of the cards from those sets are almost non-existent.) People who already played gold and silver got a third version that changed very little. It was still technically alive at this point but nearing vegetation.
Ruby and sapphire was the plug pulling. The mostly bad plastic chinese knockoff bargain bin toy looking designs. No day night, no transferring your bros you've had for four years. No charizard, no Pegachude. (At least not for over a year.)
Kids that started with rby growing up is a factor too as others have mentioned, but I dont think its as significant as other factors. If it didnt have the drifting bloated malaise and the final toxic shock of ruby and sapphire it could have survived.
t. Born in 1990