Why don't popular animes have crossovers?
Comic books do it
Videogames do it
Movies do it
American Cartoons do it
>Hello! I'm an Indian girl, you can tell because I got a dot on my forehead!
But it's more of unlike western shows being owned by the stations, anime is owned by the studio. Most of the time there isn't much way to fit a useless crossover in a series, and it's only going to revelant to have a full crossover episode like you would see in american tv if both anime are airing at the same time. This would require X studio be be doing both anime at the same time which while it isn't rare the shows would also have to have the same animation style roughly and be of the same topic.
At least those are the reasons I can think of. Anyway it took what, 15 years for the Simpsons to have a crossover with Family Guy? It's not as common in American TV either, unless the show has been airing for a long time. I remember there was a crossover with According to Jim and Everybody loves Raymond. Disney and Nick only does the crossover with other shows when one is dying for the most part.
It's relatively common in manga though like >>124122232 said.
There's going to be a Sousei no Aquarion x Aquarion EVOL crossover OVA this year