Originally Posted by Osb0rne01

Not all popular, this means Invitations are not available in common

This is a post about a specific site, and you started discussing general things, a little more and you'll get to philosophy.

Like many closed trackers they don't want to see people who came for one day and don't contribute.

Servers have a performance limit. Often they write a limit on users (Registered Users / Limit 55,951 / 60,000) based on the numbers they set the threshold for invitations. This is better than ptchina crashing forever or Bts crashing during open registration.