Because the horse had to pull up after being bumped more than once. As others have stated, the jockey for #1 deserves a lot of credit for keeping his horse and those around him from having a spill of any kind. If there is anything positive to take away from that race, it’s that.
if they went around 5 more times, none were gonna beat the 7 horse...one the worst calls in sports ever...disqualified for something that didnt happen...