My armchair guess is that because traffic hitting the cable is already serialized in some way that larger channels make sense? Of course, those large channels could also be multiplexed in some way and most long-range lines run DWDM/OTN, so I'm just as likely to be talking out of my ass.
I think the point is that fiber can have wayyyyyy more total bandwidth than any wireless technology, so you can afford to make your channels much larger (with more bandwidth). The only reason to make a channel smaller is to make room for a different channel.