The PTR server has limited capacity, in part due to the background data collection that happens. When a spot opens up, you can get in. There is only one PTR server for everyone worldwide - which is why it gets crowded.
We have had test cycles where the wait was 10+ hours long! Sadly the old forums are gone or I would link some of the complaints from back then.
The PTR is not meant to be as accessible and stable as the Live Servers are. It is just for testing.
For anyone new to the PTR - keep in mind nothing we do on the PTR transfers back to Live and the PTR can (and sometimes is) wiped back to zero even during a test cycle. What you do today may be gone by Tuesday if they do a bug fix patch and need to gather fresh test data.