The Russians simply don't have enough trucks, buses, and other motor vehicles. Says a great deal here about Russian military logistics planning/procurement/budgeting. They apparently bet everything on a quick strike against little to no resistance. Now it's a grind and the Russians know they haven't enough. Remember, the Russians must maintain forces in other places besides the Ukrainian front. Can't strip them too much. Now they must take civilian trucks/buses, with the associated effects on the economy. You can bet militaries and intelligence organizations all over are paying the very closest attention to all of this. The Russian giant has, indeed, feet of clay...