Biggest reason is that putting cable and fiber in the ground is extremely expensive for the last mile. And there's no compelling reason to upgrade existing infrastructure in an area where there's no competition.
Yeah, I really don't understand the question, techs. The constraints couldn't be more different. On the wireless side it's towers and spectrum and devices, while on the broadband ISP side it is hardwired connections to the point of presence.
That said, I am not claiming that ISPs are constrained solely, or even primarily by these issues. For example, cablecos can support a _lot_ more Internet bandwidth over existing coax infrastructure, but that service is competing with the digital video services, on demand, premium channels, etc. They want to hang onto that model, and they aren't eager to allocate more mhz to Internet so we can all go around them and get our stuff elsewhere.