I don't know VG's reasons, but one reason is that the bike path on the west side of Vail pass is freshly repaved, but on the east side it is now very rough with big bumps and cracks so that descending the east side is much less safe.
Warning - the paint they used on the Vail side of the pass gets very slippery when wet I have heard of a large numbers of accidents Also note they have just started resurfacing the wooden bridge closest to vail pass on copper side you have to get off bike and scramble . Pushing a bike i'd rather scramble uphill than downhill