It's a combination of things. Ultimate MJ bores me to tears. The whole "Best friend becomes true love" thing is played to death and Ultimate Gwen is waaaaaaaayyyy cooler. 626 MJ doesn't bother em so much as....I've grown up with it always being MJ/Peter so I always wanted to see a new person with Peter.
I like Black Cat but the possibility of that relationship working is like 1%. Gwen just seemed cooler to me after reading the Essential stuff.
To be honest if somehow, by soem insane, impossible twist of fate, someone write a PLAUSIBLE and WELL WRITTEN Gwen revival. I'd totally forgive OMD and hope for Gwen/Pete.