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One of the main goals of Pokemon was collecting all Pokemon. Gen 2 featured 250 monsters across Kanto and Johto, which could be played together. But come Gen 3? This is when it started to become increasingly inaccessible to the average player. Loss of connectivity between games meant that you had to start from scratch. There were almost 400, monsters, now split across a bunch of different games including Gamecube RPGs. As well as "Mythical" Pokemon events which were now tied to real life events and movies. Leading to lost locations like Faraway Island or Birth Island which can't be played today without some ancient ticket from the 2000s.
All this plays into why casuals stopped giving a shit for many years.