Well logistically the Ottomans were at the end of their tether in Vienna. If they win, they still wouldn't be able to push on further.
And if they won because of an earlier assault, they would still lose because now they would be besieged by the Austrian, Holy Roman and Polish armies within Vienna.
And even if they win because the Christian counterattack fails, the Turks would likely only be able to hold on for a couple of years before another 'crusade' is sent towards them. Vienna has too much symbolic importance for the Turks to stay there unmolested for long.