So there's a small matter of internal organisation, to wit: 1. Novgorod's hinterlands were given over to tax farmers from Novgorodian nobility, which hardly encouraged permanent development like Sweden did in Finland and 2. Novgorod's army was raised ah-hoc every time they went to war; they only had the Archbishop's guard otherwise in terms of a permanent military force. It's fine for short campaigns, but it makes occupation of anywhere far away quite difficult.
On the other hand, the Novgorodians did try to expand permanently westwards by building fortresses. With some changes in military fortunes, who knows? It might be that they'll encastellate enough that Sweden simply cannot dislodge them anymore.