Probably because the Superman of my long distant youth was an out and out speedster, and it really was an open question whether or not he could run faster than Barry. Undoubtedly it was close, and I don't think anybody thought any less of Barry because Superman might have been a bit faster. Certainly I didn't, Barry was one of my favourite characters, and I accepted he knew more about "speed tricks" than Superman.
But to be fair. I think very few Superman fans care whether or not Flash is a bit faster. What worries more, I suspect, is the suspicion that we might be heading towards a continuity where Flash is many times faster than Superman.