[QuadrupleA]

Can anyone with an electronics background explain why it's so hard to clock a 6502 higher than a handful of MHz, when modern chips can do 1000x that? Is it just larger transistor scale leading to excess capacitance / slower switching?

[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.

[MagerValp]

The 6502 has very limited pipelining, and every CPU cycle is tied to a memory access with no support for wait states or stalls. At 1 MHz it can work with really slow memory (roughly 500 ns), but at 10 MHz it needs ~60 ns, and at 20 MHz something like ~20ns. The architecture simply wasn't designed for anything above single digit clock speeds.