On a 300m track you only need be about 55-60s back at those paces to be lapped.
Everyone stepped on to the track with the same rules. You get lapped you're gone. If you're in the front you don't want to be having to pass people who don't belong on the the track with you over and over.
I particularly like the SEC rules where the race actually will pull off even scoring positions and only 5-6 guys might score.