Most shared hosting providers, especially the cheap ones, don’t allow you to change the Namservers of your Domain. Even though they should be happy that you would like to protect their infrastructure.
This is the same for most low cost domain hosters. Maybe the main reason is, that they are no longer have control over the domain you pay for. When you change the nameservers for your domain this will be done at the registry directly. Tech and Zone-C will change to you. They can’t easily lock down the domain anymore (but the shared web) if there are issues like unpaid bills for example. Or whatever the reason may be.
Some hosters allow you to add additional namservers. But this is useless for cloudflare and (mostly) for backup purposes if the hoster’s nameservers fail.