[raverbashing]

One reason is that: transistor size and switching speed. Though the technology of the 6502 probably could go 50MHz? 100MHz? Not sure. Would it be equivalent to 74HC TTL line? Again not sure

But the main (basic) reason is that the internal logic blocks don't worry too much about processing and arrival times beyond the speed at which they need to operate. What's simultaneous at 1MHz might be not so simultaneous at 10MHz or 100MHz

Another (advanced) reason why overclocking it might be hard is EM interference inside and outside the chip.