It's not why. Look at movies grossing as well as, or better, than FF2 since it's release and compare the competition.
Ratatouille
Die Hard 4
Transformers
Potter
Simpsons
Chuck & Larry
Next week we'll get Bourne Ultimatum, and I guarantee that movie won't suffer because of too much competition. And if you still want another example, the week after that we'll see Rush hour 3 which will probably suck but will still take in some serious cash at the B.O. So, basically, you can't blame other movies for FF2's performance being as poor as it is. At most, competition took away a couple of million, not 25% of it's potential B.O. revenue.