Because Excalibur was used as a "start-up" comic for new artists and writers instead of as a continuity-based storyline. So you get some new writer at Marvel, the boss says,"let's see his stuff, but don't let him frag up anything important." The other boss then says, "Ok, chief, how about we put him on Excalibur, no one really pays much attention to that one anyway." And the boss says, "Great!" Hence, Douglock, Wolfsbane, Captain Britain becoming super-scientist for the last 6 issues, Kitty Pryde Agent of S.H.I.E.L.D., and of course Kitty and Pete breaking up.
Gabriel