Price! C64 cost £399 at launch, while the top of the line Spectrum was £175 (and there was a cut-down model for £125). In today's prices it's around $500/700 for the Spectrum vs $1600 for the Commodore. And the UK was considerably poorer than the US in the early 80s too. The Commodore certainly did more, but entry price really mattered.
Thanks, I couldn't find a usd dollar amount for the spectrum. I did know the c64 cost $1600 in today's prices (that's a good self-built pc today, not quite gamer specs but still decent)
The Spectrum wasn't released in the US. However, the semi-compatible Timex-Sinclair 2068 mentioned by others was. It was quite cheap, selling for under $200 in 1983 (about $570 today). However, it got very little support and software in the US and so it didn't do well.
c64 famously had its price cut very aggressively multiple time shortly after introduction, as commodore tried to drive texas instruments out of the home computing market (they succeeded, with atari and the rest of the industry becoming collateral damage). I believe not even a year later it cost half as much.
[All prices from Wikipedia]