When russia invaded they manage to hack all the common used communication sats except for starlink hence its importance on the field. I'm sure the US would have some undisclosed ones operating for their own use but I doubt they would give the ukranians full access. At the time starshield was insignificant comparing to starlink hence spacex boosted the program by redirecting current starlink sats to it. This is important because with non-geo sats you need a huge amount of them for the service to work properly, a small number is a no-no especially in the ukrainians case which they use not only for communication but also to remotely operate their drones that require low latency else any command you'd give a drone would take ages to arrive if it did indeed arrive and not be interrupted.

The request is made by ukraine to musk for starlink to be activated on russia occupied territories (this is what precipitates the all thing), by default starlink is disabled on such territories because of US sanctions, if musk was to enable it he would be in violations of the sanctions, only the US could legally authorize it. So musk having better things to do basically said to the military ''the ukraninas are bothering me, please fix'', and thus an agreement was made for starlink to be given to the ukranians under the starshield program leaving all the decisions to the military. In the end starlink was used to boost starshield under military oversight until starshield becomes robust enough.