IMHO it's because ISP is a much more broad term. When I connect from my University, I have to either use TLS or trust every step of the network from whatever co

IMHO it's because ISP is a much more broad term. When I connect from my University, I have to either use TLS or trust every step of the network from whatever commerical ISP they use, to the whole network infrastructure they built to the building, to the other devices using the same router as me. When I trust Cloudflare, my data is encrypted whenever it's not directly in their hands eliminating those other steps.