It’s mostly a question of desirability.
Back in the 90s, for all their size, Marvel and DC were both run like “Mom ‘n Pop” shops, Whereas now, both companies are part of larger conglomerates with larger concerns.
On the most basic level, what incentive is there to do, say, BATMAN vs IRON MAN, which will cost the same to make as any other book that we’d publish, and from which we’ll only receive half of the profits? We could put out any other IRON MAN book, and DC could put out any BATMAN book, and in all likelihood do better—even if the overall sales aren’t as good. To say nothing of the complications of the collected edition, and the digital rights, and so forth.
The only real reason to do big crossovers is for the fun of it.
Seems like a pretty good reason to me