Well the problem with the cobra is you bleed energy rapidly and in a dogfight that usually means death.
A bad guy on your six will probably have shot you already if you've had the time to realize you had someone on your six and should do the cobra manouver. It's faster to fire off a sidewinder or let off a burst of cannon fire than to pull off that kind of move.
Not to mention, if you do pull it off, you'll in the sight of the enemy while you do it, while you will lose sight of your enemy until your nose pitches down. Very dangerous.