Putin is apparently lobbying hard for Belarus to enter the war on his side. It would be a form of escalation–the creation of a second front, presumably–without running the risk of a NATO attack on Russian forces. Why not try it?
Because Lukashenko may be a tyrant, but he’s not a complete fool. He doesn’t have anything like the control over his country that Putin has over Russia. Entering the war, with no obvious gains for Belarus, would undoubtedly rouse an already unhappy populace and endanger his hold on power. Worse, NATO might feel free to attack his military and even his cities, given that he doesn’t have nuclear weapons. The potential for disaster is obvious; the second front might be in Belarus itself, not Ukraine.