I'd wager it's because it seems like a simple concept on the surface; create a hundred or so funny looking monsters with special powers and make a game around capturing and training them, but in reality while Pokemon looks like a childrens game from the design aesthetic it has a lot of complicated math and strategy behind the scenes that contains an entire autistic competitive scene frothing at the mouth. I think a lot of possible competitors that try to step into Pokemon's shadow don't realize that if they want to really nail the delivery and become a rightful competitor to something like Pokemon they will have to nail the competitive metagame design from the outset, because the market simply won't tolerate a game needing 2-4 iterations to really hit it's stride to keep up with it's direct competition that is already established.
Let's be real as well, the competitive gameplay is what really matters as well. That's part of what gives Pokemon the staying power it has. Without that, a competitive title won't have the sticking power to really flourish and being able to hit that kind of a development requirement in an absolute vacuum from a fresh IP is what I would imagine to be a soulcrushing challenge. False starts that fall apart probably start to have that realization, or just the complexity of actually designing something like that, and fall apart because of it.