I think everyone was exhausted at that point. Either he was too tired to morph, or he just passed out like that.
Marco had slept half the night before David attacked him, so I figured that he should have had enough energy to morph once he woke up after David had knocked him out.
I assumed that the statement that it had taken Marco the rest of the night to get free meant that he had been actively trying to get free for the rest of the night!
I've read of Animorphs morphing and demorphing while on the point of death. What's a little lack of sleep besides that?