Related question: why can't people apparate to platform 9 3/4
A: some can, but many can't apparate (including the muggles that make it onto the platform), thus it can be assumed that not all can apparate away either.
So was the platform blocked only for Harry and Ron? (How did Dobby know Ron would be with Harry?)
Were the people on the platform "stuck"?
did anyone even bother to attempt to leave?
Would Harry and Ron see people leaving the platform? (this brings up it's own question, how do you make sure people entering and leaving don't run into each other) - or do you exit via some sort of other wall, perhaps 9 1/4? - in either case, was there really no other magical person they could ask for help at the time?