It isn't clear why they have stopped but it may be breakdowns, mud, logistical issues, drivers refusing to move, almost anything. But it is surely not deliberate, unless Russian generals are artfully rebelling against Putin's insane orders. If Ukraine had an air force, the entire convoy would be toast.
It may be wishful thinking on my part, but if, as some reports suggest, the convoy is short of supplies of food and water, perhaps in a few more days they will have to surrender.
Now that would be a thing. Actually, the best thing, because Ukraine would have all of that equipment for themselves. And Putin would have had the sort of disaster which would make it relatively easy to remove him from office.
How nce it would be if Russia was a friendly neighbour.