So the opposition to Assad is boycotting the Syrian peace talks and ISIS isn't invited. So that means none of the ground forces opposed to Assad will be at the conference. How can you have a conference to resolve a conflict if only one side shows up?
(I know it is not technically true in this case that only one side is showing up. The conflict has too many sides to ever say anything that straightforward! And the foreign powers like the U.S., and various NATO members will be there and they are anti-Assad, anti-ISIS and pro-some of the anti-Assad factions. But the conference is not going to resolve the conflict unless the main parties to the conflict are at the conference.)