They never had the chance.
Doc took Marty home and travelled through time. Then the rest of the trilogy happened.
If you meant why didn't they get out of the van and run up to Doc.
The crash probably knocked them out.
Since Marty accelerated up to 88 mph and they were tailing him well enough just before that we can assume they made that crash around....considering they hit the breaks..50 mph per hours? 100 km wow that's serious...
"Usual? You lead a pretty unusual life, Doc." - Marty McFly
Golden Age Whodunnit/Locked Room Murder Mystery fanatic.