Yeah, I wish there was more information about it. At the time, my wife was working on his campaign, so I paid a lot of attention to the positions he was taking on various issues, but I don't recall many specifics. From what I can remember, and from likely the same google results you got, they seem like a fantastic idea. So I have the same questions about why there hasn't been significant adoption. The only guess I have, is that while cheaper to install or maintain than new concrete installations (that are then properly maintained), it is likely a lot less cheaper for municipalities to do nothing at all, which gets us the terrible sidewalks that we have today.
Indeed, one of the selling points for rubber pavement is that it is porous, and therefore better for maintaining ground water levels and eliminating drainage issues.