I wonder if that's in part or related to our conversation on writer's not being able to make a big enough splash; why sign a guy who may not end up being a big thing?
Could very well be the case, but I think, if I remember correctly (which truthfully is very doubtful

) both companies did a pretty decent job only signing only the top tier artists and writers. I think the reason they stopped doing it was it became too ineffective when it came to cost vs impact in sales. They were giving these contracts, complete with health insurance and all the bells and whistles, and after a few years of seeing that the books didn't sell "that much more" to counter the cost, both companies decided to drop that aspect of contracts.
I am not sure, of course

, but I believe they are starting to bring the exclusive contracts back, but this time in lieu of including health coverage and all the other bells and whistles, they are just paying out a higher rate with the promise from the creator to stay exclusive for a year or two or whatever.
However, if a book they are on still doesn't sell well enough ... and give them the time to work those sub-plots, Marvel or DC can just pull them and put them on another title that needs a boost or one they plan on starting at issue #1.