It's my understanding (though I'm far from an expert) that vehicles aren't convenient in a lot of places around the front because the roads aren't great (and weren't great to begin with even before the shelling/bombing started) and because the springtime mud conditions in Ukraine can be borderline impossible to drive through with both Western and Russian vehicles. There's no short supply of photos online of all kinds of tanks/transports/trucks from both sides completely stuck in (and even rescue vehicles coming in to pull them out getting stuck as well). So it doesn't really surprise me to hear news of donkeys or other animals being used taking off around the time the seasons are changing and winter is ending.
It wouldn't make much sense to pay big bucks for a hummer/mrap/apc style vehicle for short hauls if it can't even make the trip without getting stuck in the mud and getting hit with a drone.