Because it would only create hate/panic.  If they were terrorist it does not matter from witch country they are because they do it for motives like Religion/Politics..

What if those terrorists end up being Danish, or French..Attack those countries?

Probably because no one was hurt and there aren't any ties to groups conspiring.

9/11 - lots of people were hurt, information showed that it was planned by someone, most commonly blamed is Bin Laden's Al-Qaeda but some conspiracy theorists argue otherwise.

2005 London bombings - lots of people hurt, I believe a group claimed responsibility, don't know a lot about the incident myself.

this incident - no one hurt (the big reason), probably some idiot shooting at the pentagon without realizing that the glass would be bullet proof. Not enough information to really make a decision and no one has claimed responsibility also would contribute to not declaring this an act of terrorism.

EDIT:

To further expand, things like the Columbine shooting (or the more recent university shooting that I can't remember which campus it was on) aren't considered terrorism (mabye some do as they are acts of terror) because they can't link the attack with some group. Once the perpetrator is caught, killed, or commits suicide there is no one else to fight and there are no terrorists to go after.

Muslims pray 5 times a day not 3.