Well, in short, any web site with any money to pay the ISPs is likely already running on a CDN that's been paying the ISPs for a decade. The portion of that market that's not yet monetized is likely very small. Besides, slowing down general-purpose web traffic makes it look like your service just totally sucks ass.

But more generally, slow-loading websites make the ISP's service look shitty. There's almost no cost for them to just QoS the low-bandwidth stuff up and it makes people think their service is better.