Two things, most likely a mix of lack of top end, and not being close enough to the wheel in front of you. If you're close to the wheel in front, jumping is a lot easier (ever looked at a power file for a lead out versus the sprinter?). The other is probably using too much energy during the race to stay way to far forward. You said they're 20-25 people in the race? You could literally sit in last place the entire race, and just move up for the sprint. Even strung out that's not a lot of people, and there is almost no incentive to stay close to the front.