Because they can't:
a) Ukraine's lines of communication are much shorter, so they can move troops around more quickly than Russia can assemble them in the north which would be detected almost immediately;
b) Russia would need Lukashenko's cooperation to launch a second attack from the north. Not sure that would be forthcoming.
Russia has moved some assets back into Belarus and has recently launched some air attacks from there.
“Russia would need Lukashenko’s cooperation to launch a second attack from the north. Not sure that would be forthcoming.
Russia has moved some assets back into Belarus and has recently launched some air attacks from there.”
Noted. Thanks.