Comcast is selling more speed to folks than they can possibly keep up with. As more folks in your area come on line, the speed for everyone goes down. Plain and simple. If your neighbor is paying for way high speeds like 150mbps then you suffer. He's on the same cable you are, correct? They did not run a special cable just for him did they? If order for them to raise his speed they have to take some from you and your neighbors.
There is only so much bandwith to go around. That's just how it works.