Memes

Pokemon is how a lot of people got into games to begin with. It was a new and innovative experience from their perspective. Pokemon Red/Blue was a competent game with some fresh ideas, but through luck/marketing it became the launch point for a massive population of people into the gaming industry.

So now you've got a few factors playing into Pokemon hype:
Nostalgia (you never forget your first)
Production value (this made money, pump more money in)
Incidentally a formula that favors expansion (just add more Pokemon)

These factors are enough on their own to carry a franchise for a while, especially for an otherwise ignorant audience that doesn't play anything else (just like the people who just play FIFA games and nothing else). But at some point, it becomes too obvious even to the most zealous supporters that the formula is, well, a formula, and it's not changing or improving, and even they finally begin to criticize the product. It's easy to have a favorite pokemon out of 150, maybe even 450, but now there are over 1000 and it becomes exhausting even for die-hard fans. Even the number of types has exploded to 18 without actually having any interesting interactions to justify them, it's just more for the sake of more.

Plus, the most recent releases have been impressively lazy, again so much so that even megafans can't nostalgia their way out of it.

All this together makes for a history of a franchise that was one vehemently defended but is now seen as an embarrassing phase one went through as a child.