Because that shytty story required that every character act like a moron so that Marvel could get to the already decided on ending of Prof. X being dead, Cyclops being a fugitive and the Avengers and X-Men merging.
Your right it was bad writing. I get that you want the x-men and Avengers to fight now that both got super hero arimes but after the x-men had the phoenix non of those guys didn't use any of infinity gems? No one went to look for the ultimate nullifer? Nothin...........