There is a big big difference in Yemen and Syria , You can not find one single foreign force fighting against Hadi in Yemen and the whole job is done by local people but in Syria the only reason the war is still going is foreign forces who are doing the main job otherwise the so called moderate rebels had been wiped out years ago .
Assad is not a good leader for sure but he has supporters , more than half of his army are Syrians and add millions of Shias , Alewites , Christians and Druze people to them and this is why Assad hasn't fallen after 4 years against hundreds of thousands of rebels from more than 100 different countries .