Primarily the two river gorges their tanks have too drive through to get into the town from the northwest. They could approach from the southeast and southern direction rather easily, but that movement would be spotted. The main factor that stopped them probably was the 5 Presidents from Eastern Europe. Russia was not quite ready for war there at this time. Perhaps they will be in a month or two. Since Russia could not finish their initial end game, they have been stalled and are playing a middle game. Using the Free Georgians in Tbilisi as hostages. IMHO. Unfortunately, they could tire of the middle game and finish the way they wanted to finish anytime now. The west seems to only have pawns on the board at present.
That is pawns in the literary sense, not the chess sense.
Primarily the two river gorges their tanks have too drive through to get into the town from the northwest.
There is a paved road from the northwest that comes right into Tbilisi.
The main factor that stopped them probably was the 5 Presidents from Eastern Europe.
The five Presidents have come and gone.
Russia was not quite ready for war there at this time.
I believe Russia does have the troop strength and the equipment to drive into town and take Saakasvili.