It might have something to do with skill. People have been penciling 1080s for a long time, but it takes extra skill to grab. If you aren't grabbing, it shows that you are not as good. Off of rails though, there is very little time to grab. As a result, spins take over.
it's just trends and what we consider cool. sort of arbitrary in sort of the same way "we" don't think straight back fulls are cool compared to corks.. and grabs off rails are not "in"...not to mention the fact that if youre doing a 4 or a 6 off a rail, you have somewhere between 1 and maybe 20 feet usually for your spin.. if youre doing a bigger spin off a jump you have waay more airtime
Usually the air distance off a rail is less than 5-8 feet. With such little air time, it looks awkward to reach for a grab, touch, let go and land. Even on a jump with 5-8 feet of air grabbing usually looks awkward unless it's an easily reached grab, like tail. If it's a canon rail with 15+ feet of air, grab away.
i dont necessarily think that people need to grab out of rails more, but i do think that it would be good to incorporate more presses, different types of slides, and other creative things into rail tricks so its not just all spinning