If you look at how many truck are needed elsewhere in the country, and how many are bringing in different stuff (Class 1 to 9) You would see only a small amount of truck are available on one sector of the battlefield. It may be only a small amount of trucks are available for this task.
And then you also need to consider a few different thing.
1.) If I bring all the guns in, how many ammo can I bring with the truck? Do you want to bring all (or at least as many as possible) guns into battle but wait for 4 days (2 days each side) to bring the ammo up? Or you want to bring half the truck with guns and half with ammo, so you can at least put some guns in action?
2.) What about convoy security? Don't forget the same thing happened to the 40 miles Russian convoy in Kyiv can happen to Ukrainian when they bring in a large quantity of supply. You need to hide them during daylight somehow, Russia have satellite too, they can see if you have a long convoy bringing in big stuff.
3.) You don't just have that on the road, you still have civilian traffic, military traffic, other supply using those road, you don't want to jam up the road for everyone, and you will when you bring in heavy equipment.