I'm not sure what stock car meetings you are referring to, but crowds are very variable depending on the formula and the event.
Yes, the F1s get biggish crowds, especially for the more important meetings, but how many of them are locals?
F1 stock cars is a 'travelling circus' with drivers and fans coming from all over the country. Many F1 fans will do most meetings in the season, irrespective of the location. Team sports such as speedway are very different and rely quite heavily on local support. How many speedway fans would travel from Lancashire or Yorkshire for a meeting at Kings Lynn ?
Stock cars is my main hobby and I will generally watch 40ish meetings in a season from my Nottinghamshire base. My nearest track is an hour from home, and most are within 2 hours that I visit reguarly, but I will also visit Taunton once or twice a season at around 4 hours, and will usually do a St Day (Cornwall) or Cowdenbeath most years and these are around 6 hours away.
Speedway fans won't travel like that generally, with the bulk of the crowd at most meetings being fans of the home team.