Bonus reason: The spectrum was a fraction of the size of an AppleIIe, about the size of a small book. But the rubber keys did suck.
Not saying Spectrums weren't awesome (I don't really know; the only one I ever owned was that 1k first-gen model with a 16k expansion; it was miserable), just that you kinda have to compare Apples to...umm...not Apples.
And, of course, the C64 is the bestselling personal computer of all time.
(I should admit that you're arguing with an 11 year old SwellJoe here...I loved my C64. And, crazy as it sounds, I also bought some C64s on eBay over the past few months for use in composing chiptunes. Thus, evidence indicates I might be mentally unstable with regard to the C64. Backing away slowly might be the best option.)
Also, because of the popularity of the platform, developers knew it really well, and were able to utilize tricks to occasionally go above and beyond expectations (for example, it's possible to get a lot more than 16 colors out of the C64, particular in still images). These undocumented modes and techniques became pretty common knowledge through the several magazines that covered the platform, and through various demo/warez scene channels (like bulletin boards).
1. Us poor folks could afford one.
2. Everything the Apple IIe did, the C64 did better. Better games, better high res graphics modes, dramatically better sound, and more software (because it was cheap, it sold more units, bigger market share, more third party applications).
Bonus reason: The case was a third the size of the Apple IIe, making it much easier to haul over to friends houses.