Because that security model means that a compromised ISP can permanently MITM you the first time you visit a site on any computer for the first time. In exchange for losing the single points of failure, it creates a constant stream of opportunities to break the trust model.

You may think that's a good tradeoff. I don't, but reasonable people can disagree about it. However, it's completely untenable for financial information, because attackers after financial information aren't laser targeted and are perfectly happy with a constant stream of compromised sessions. Remember what TLS was designed for in the first place.