I think it all boils down to the fact that arcade-style games are dying, not so much a particular genre. Most ppl only want action adventures/RPGs, BeMani, vast simulations and FPS games, with a few 3D fighters thrown in, oh, and racers, that's it.
Developing games has gotten very expensive, and most companies go by the motto "better a good clone than a crappy new game concept". They fear to come up with something that cost a fortune to make but sits on the shelves forever, so they take game ideas like Halo, the Final Fantasy-series and a few real-time simulations, make a new game out it and release it under a new cool name, then the cycle starts again. That's not exactly a new thing to be honest, game developing companies do this since the very beginning; just look at the insane number of clones/boots of games like Pac-Man, Galaxian, Space Invaders, Pong, and later on Final Fight, Mario, Capcom's SF and SNK's KoF series, etc. etc., not to mention all the similar generic horizontal or vertical shooters that had been released by the dozens every week in the 80s. I clearly remember a Zapp!64 "day at the arcades" article where ppl constantly complained about "the shmup craze" and the ridiculous number of Mario-look-a-likes... vg history repeats itself, only with a lil' more processing power...