At the very beginning of chapter 7 in part 1, in the very first sentence, Winston gives at least one of the reasons that he believes the Proles are the only hope at successful revolution. He mentions that they are at least "85% of the population," so, they have numbers on their side. Because there are simply so many of them, they are the only ones that could be a large enough "force to destroy the Party." He said that rebellion was so difficult under the watchful eye of Big Brother, but that the Proles who had more freedom from that ever-watchful eye, could simply unite by "rising up and shaking themselves like a horse shaking off flies." So, by the mere fact that there are so very many of them, all they would have to do is rebel, and their numbers alone would ensure success.
Secondly, they aren't under the oppressive eye of Big Brother as intensely as non-Proles are, so they have more freedom to move about and form a rebellion without being detected. Because of this, they are more free; Winston envies that freedom a bit, in fact, and wishes they would use it to overthrow their oppressors.
Unfortunately, through very clever and insidious propaganda and constant war-mongering to elicit a fury of patriotic devotion from the Proles, they never do rebel. They are constantly pitched to a fever of rallying to the cause of the nation in wars against its enemies, and the propaganda. The masses are so controlled by this, in fact, that the Party has no worries whatsoever about the Proles. Their busy workschedules, their exhaustion, and their being brainwashed into devotion to the cause, all keeps those masses in check.
I hope that helped; good luck!
I think that there are two reasons why Winston thinks that the proles are the only hope.
First, there is the fact of the sheer number of proles who exist. According to Winston, they make up 85% of the population of Oceania. So there are enough of them to actually make a big difference.
Second, the proles are actually still more human than the Party members. The proles still get to be free because the Party thinks that they are not important enough to try to regulate. Because the Party doesn't observe them, they still interact with each other the way that people do and they still have more in the way of human emotions than the Party members.
So the proles have the manpower to be able to rebel and they have the humanity that is needed to want to rebel.