[vcf-midatlantic] What limited the C64's CPU to ~ 1 MHz?

[John Heritage]

Does anyone know the reason why the C64 only ran at ~1 MHz?    Was it to
keep RAM costs or system cost as low as possible or was there a different
engineering reason?

(The Atari and BBC machines managed higher clocks, but were very expensive
compared to the C= devices, although the 1977 Atari 2600 managed ~1.19 MHz
out of it's 6507 (6502 with low pin count/less addressing)..