I remember when The Broad Street Run in Philly went to a lottery system. It made sense because it is a one-off race; i.e. there are no "challenges" with it, and there's usually about a 75% chance to get in. They determined that there are about 40,000 people looking for 30,000 spots.
But rD is different, there are the different challenges, and the C2C. We can't just have rD say "nope, you lost that lottery" That would rather suck.
Not only that, but rD doesn't care which 20,000 people get a registration, just that all the slots are filled. If it's first-come-first-served or a lottery, they don't care, they get their money. And making it a lottery is more work for them for no more profit.
Now imagine a bidding system for the races