Why do some revolutions succeed, while others fail?

Obviously, some revolutions are more effective than others. Some are totally still born, some result in massive changes. Why exactly? Obviously, it's necessary for a large proportion of the population to want change, for change to occur, but, quite frequently that isn't nearly enough to cause a successful revolution. In 1989, 100 million Chinese were actively demontrating for democracy, they were crushed completely merely by Deng Xiao Peng -- who wasn't really a communist hardliner at all, just a pragmatist -- killing perhaps at most a few thousand students, in the Tiananmen Square massacre. Everyone just stopped demonstrating, end of story. A similar result occurred in Iran just recently. Massive demonstrations involving many millions, a few thousand killed, everybody gave up. Obviously, it helps if there is some outside assistance. The American Revolution probably would have failed, if France hadn't massively intervened, particularly with naval assistance, to her great cost. Actually, this bankrupted France, and caused her own Revolution! But, that Revolution could quite possibly have been crushed, if Louis XVI had been tough enough, and acted quickly and firmly enough, with military force. The Russian Revolution of 1917 was caused by institutional collapse in the face of wartime deprivations, and an obsolete social structure. So, the key points may be the weakness of the actual government, and the extent of the deprivations really suffered by the citizens themselves. Also, the extent of substantial outside assistance, opposing or supporting the existing government. Iran has solid support from both Russia and China, and, its social system, however disagreeable to the West, is tried and true in terms of surviving any and all conflicts, with anyone.