From what was said in and implied by the episode, life support was down to low power. The crew couldn't survive aboard the ship.
It very well could have been a no-win scenario.
We get this line early on in the episode while exploring the Constellation:
KIRK: But where are they? I can't imagine a man like Matt Decker abandoning ship while his life support systems were still operative.
What I gathered from the dialogue was:
Communications blocked
Can't run (Enterprise with Warp Drive could outrun the DDM so that's probably because the Constellation's antimatter was inactivated by the DDM and the warp engines were destroyed)
Fought at a distance with phasers and were ineffective
Impulse controls were fused solid. No impulse control. Also another reason they couldn't run. (There was still enough for 1/3 impulse power when controls were swapped with the warp controls.)
Life support still operative and Kirk didn't see that as a reason to abandon ship.
Transporters worked until the Constellation was hit one last time.
So to me the reason to get off the Constellation had to override any reason to stay on the Constellation.