Why requirements for upgrading accounts?

Why are there requirements to upgrade an account?

I want to pay for an upgrade, but have to have at least 50 posts. What is the reason for this?

You can start with prime, it only requires 10 posts. For the rest you need, besides the posts, to have a positive reputation, so the most sensible thing to do is to start by upgrading to prime

But why do these requirements even exist? Is there a reason for them?

Because the quality of the user is given priority over the money theycan earn from the upgrades, I imagine. Otherwise it would be full of users who can pay for the subscription but would not contribute anything. (I'm not saying that's your case)