The political analyst Ian Bremmer said Musk told him he denied Ukraine's request to extend Starlink's range, fearing the potential for escalation.
www.businessinsider.com
As far as I can tell there's two different Starlink situations going on in Ukraine.
A blackout over certain rapidly expanding counteroffensives. The explanation is that Starlink has a blackout over Russian controlled areas as a precaution in case the equipment is captured and Ukraine has reacquired territory faster than Starlink can react. Seems plausible enough.
The other situation is that Musk doesn't want Starlink to work in Crimea because he doesn't want Ukraine to retake Crimea. But to be honest I think Crimea is going to be the last territory Ukraine retakes if they retake it at all.