By "attendance", you mean physical, actual spectators at site? (sorry, English isn't my native tongue). If so, that's more or less insignificant when you're airing live races to million of viewers all over the world. While the track itself gain $$ selling tickets to attendancees (as I assume you mean), Dorna, FIM and other get umpteen times more selling the broadcast to networks all over.
The number of live online viewers TV/Internet compared to costs is crucial. If it's 50,000 spectators at site will of course make the TV broadcast more interesting than if it were empty stands. But there are tons of broadcasts captivating millions of viewers without any "live spectators" at all. TV-shows, movies, etc.
Sorry for ranting......