Full disclosure upfront, got tickets last year but failed this year despite multiple people trying with multiple devices. I spent most of this morning unable to reach the queue page.
My thought is this, most of us use more than one device to attempt to get tickets which obviously pushes more traffic onto the site than required. To alleviate this strain, why not require the registration number to be entered before joining the queue and block multiple uses of the same registration number?
This way nobody can have multiple devices and inflate traffic as you would only be able to enter the queue once with your registration number, hopefully making a smoother experience for all.
I'm aware that Glastonbury and Seetickets don't really care about this as the festival sold out anyway.