The anime universe is very rich with deeply detailed characters and experiences. It's hard to find two anime that really tell the same story with the same types of characters. Even subtle nuances between characters can lead to wildly different stories and outcomes. That's why there's so much great anime content out there!
We all have our favorite anime features and characters, and even when series' creators decide that it's time for their series to end, fans are left wanting more. It's true that it's better for a series to go out on top rather than burn out, but that doesn't make the actual parting any easier.
When that sort of situation comes around, most fans will take matters into their own hands. Or...own minds, I should say. Why let those characters die off whey they could come together in an anime feature of their own? What better way to celebrate great anime characters than by putting together a crossover?
A lot of anime creators are into the idea of crossover between popular characters, which is why we see this trend continue on today. If you could have a pair of anime characters or casts come together in a crossover, what ones would you like to mash up?