Also, orbital bombardment with phasers doesn't always involve pinpoint accuracy. We see fairly sharply focused beams when our heroes demolish Apollo's temple or Vaal, but only diffuse glow when they bombard the Iotian gangsters, or when Riker witnesses the Space Jellyfish attack against Farpoint and likens it to phaser bombardment.
Personally I agree it was due to them only wanting to open the door without damaging the passageway beyond, though. This was precision work; the pedestal device probably couldn't have "leveled a continent" even though the ship had the required oomph, but the device had the best odds of making a hole in that door.
(Incidentally, that device would probably make for a piss-poor weapon, as it is only capable of line-of-sight work but seriously lacks in mobility, operator protection, or even in the ability to aim the beam past obstacles or self-protection embankments. Is that a modified camera dolly the double bubble business end is resting on?)