Several reasons. First, UN peacekeepers aren't peace makers. They aren't set up to stop wars, but to police the ceasefire when the war has ended, to keep it from flaring up again. None of the Syrian factions is ready to accept a negotiated settlement. They aren't going to do this as long as they think they can still win total victory. Why bargain for half a loaf, when you believe you can still get the whole thing? Finally, several of the Syrian groups have goals that can't be reconciled. I really doubt they'll ever be able to sit down and govern the country together. A more probable outcome is a ceasefire in place, with each faction keeping the territory it controls. That would allow foreign peacekeepers to enter Syria and interpose themselves between the armed combatants so the peace can hold. Theoretically, this can work, but all sides would have to agree on the composition of this neutral force. Often, they have trouble selecting a country that's willing to contribute troops, and is trusted by all sides to be impartial.