Wrong, the X-men books were selling
more than Avengers at that point. Uncanny X-men featuring the Extinction Team was Marvel's best selling book. Look at the sales for the month before the event, February 2012:
Uncanny X-Men 6 $3.99 Marvel 59,834
Uncanny X-Men 7 $3.99 Marvel 59,200
Wolverine And X-Men 5 $3.99 Marvel 55,788
Wolverine And X-Men 6 $3.99 Marvel 55,341
New Avengers 21 $3.99 Marvel 54,105
Avengers 22 $3.99 Marvel 52,683
Avengers 23 $3.99 Marvel 52,222
New Avengers 22 $3.99 Marvel 51,235
Uncanny X-Force 21 $3.99 Marvel 50,654
Source