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

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)..