A big improvement over Scary Movie 4. Why the low rating?
Scary Movie 5 had a few more Seltzer/Friedberg styled gratuitous references, but unlike 95% of part 4's jokes, what Scary Movie 5 had were actually... jokes, with punchlines and relevance to the spoof, rather than characters saying stupid crud and getting wrapped up in uninspired slapstick/scat humor whilst going through the motions of a plot. Most of the parodies gelled together better as well.
I hope that, like most of the other Scary Movies, this one's rating will go up in time. Seems to me the odd-numbered entries in this series tend to be better for some reason.