Since we use real world robots and courses, we only have room for a finite amount of people at any time. Imagine thousands of people all trying to fit into your favorite restaurant with 100 seats. As such, we are continually making improvements to make the experience better.
In the meantime, your best bet to get into a round is to just keep trying as often as you can. You should get into a queue within a couple of minutes.
The team is working really hard to build more robots!