The most common speed of the 6502 was 1 MHz, allowing the use of 2 MHz RAM chips (half the bandwidth went to the video chip to refresh the screen).
In 1982, the BBC Micro shipped, with double speeds across the board. That meant it needed 4 MHz RAM chips, which at the time were quite scarce and expensive. Presumably the extra cost was what discouraged other manufacturers from following suit.
Is there a figure available for exactly what the cost differential in the RAM chips was?