They can't march very well through the mud and become too easy targets for my airforce. If I set a single stack of Tacs on logistical strike in the Memel region it takes them more than a month to reach far enough to attack Riga, losing several divisions to air bombardment on the way; if they attack after the mud dries they can have Riga in their hands in about a week, with or without logistical strikes. In the end, attacking a month later means having advanced further at a far lower cost by June.
I don't think it matters much against an AI USSR, however.