I mean theres a few reasons.

Theres what Richard Stallman says about CF

Also they dont seem to shut down sites that host large amounts of misinformation

Theres also the fact that CloudFlare is centralized, and when CF goes down, large portions of the internet goes down. The fact theyre a large US company that can see what anyone on the internet does on any of the many many websites they host (its something like 60% iirc)

A large portion of the reasons are in this article: https://expertbeacon.com/why-cloudflare-sucks-an-in-depth-look-at-the-perils-of-an-internet-giant/