And the idea of the pre-registration day is just a carry-over from all the previous years of the con. When I first started going, there was a little booth upstairs in the Sails Pavilion that you could walk up to at any point before the end of the convention and buy your four day + preview night badge for the next year. And you even got a discount for doing it that way. The first three years I did it that way I paid $75 for my SDCC badge. Then it went up to $100.
But as it started getting busier and busier, and selling out faster each year, it also got harder to get your badge for the next year at the convention. They finally had to move it offsite and they sold a limited number of badges for the next year each day. And if you weren't in that line by around 5:30am, you probably weren't going to get it. I had a friend get capped at 6am, when the badges didn't even go on sale for another few hours.
So since that was no longer a viable option, now they do the pre-registration day online. So it's literally just an updated version of something they have been doing for probably almost as long as the convention has existed. And it's also a pretty common practice for conventions. For example, I was able to buy a fifth row seat to the Supernatural convention in Pasadena this year before I left the one in Burbank last year, and the Pasadena tickets still aren't even on sale yet.
As someone already noted, the pre-registration day is still not a guarantee. It's the same kind of luck of the draw raffle that the open registration is. And it's only available for people that bought the normal tickets the previous year. Despite the fact that I have been going for as long as I have, I've never been eligible for the pre-registration day online because I've also never been able to get picked out of the waiting room for the Open registration, and always have to find someone I know with a booth that has an available Exhibitor's badge I can buy from them (which costs $200 more than a regular four day badge).
While they have more than outgrown the San Diego Convention Center, there also isn't any convention center that could hold the number of people that actually want to go. So even if/when they move to a new location (since San Diego axed the expansion plan), we're still going to be stuck battling for tickets and hoping for a bit of good luck while we watch the spinning blue dots in the waiting room.