Both Marvel and DC have grown over the years from the “Mom and Pop” shops they started out as into much larger conglomerates. As such, the goals and objectives of each company have changed over the years. The truth of the matter is that there isn’t really any great benefit to doing a crossover apart from fannish enthusiasm–you can sell some books, sure, but there are other ways to sell the same amount, and in those cases you don’t have to split he revenue with another party nor jump through the more complex editorial hoops of the story needing to serve two masters. In the days when Marvel and DC were more fannish in general, this happened more often (though even then, not really all that often.) And today, it’s a much more difficult proposition. So it’s not out of the question entirely, but it is seriously unlikely that we’ll see more Marvel/DC crossovers any time soon.