Thats only because there are 10 teams in the championship which amounts to 20 drivers. If you've been watching formula1 as long as i have then you'll remember around 12 years ago there were 26 places for the cars on the grid, yet 34 drivers (17 teams) turned up each GP weekend looking to qualify. They had to get within 107% of pole position to qualify as well only a few years back, so they used to have to qualify, just like the speedway. But as with everything money has ruined the sport, hence the lack of teams in F1.
Also speedway cannot be compared to F1 due to formula1 having been run like the way it is since 1950 (if you want in you set up a team and pay your entrance fee). Speedway since 1934 up until 1994 has always had a qualification system that allowed every rider the chance to go to the world final, now though thats changed due to money talking rather than commen sense.
Jim.