The Speccy was the right computer for the times. It arrived very early on in the home computer boom and was very keenly priced, so it maintained good network effects throughout its lifespan. The machine was cheap, the peripherals were cheap and there were loads of games. Sir Clive didn't fragment the platform by introducing incompatible new models, he just kept cutting the price. The Speccy remained popular into the tail end of the 1980s, almost entirely because of the huge back catalogue of games. We loved it in part because it was a bit crap and because Sir Clive was such a hopeless businessman - the British have a terrible soft spot for underdogs.
The CPC didn't sell well as a home computer, but it was quite attractive as a small-business machine - the monochrome display was cheap and reasonably crisp, the 3" floppy drive was far cheaper than 3.5" drives and it would run CP/M. Lord Sugar responded very shrewdly with the PCW series, which was tailored for business and sold far better than the CPC. Marketing the PCW as a word processor rather than a computer and bundling it with a printer was a minor stroke of genius, with the brilliant slogan "more than a word processor for less than most typewriters".
Amstrad had the last laugh by buying out the rights to the Spectrum, but the sheer quantity of Speccies that Amstrad sold is a testament to the platform.
But it was, as you mention, technically much less capable. That said - I never really got the impression the communities were at odds util modern times when it became a thing.
Several people I knew growing up had both for example!
Then again, I was an Atari guy so felt they both were not up to the standard my Atari 800XL set ;)
Spectrum fan here, but I do love the CPC's eye popping colours :)
You've completely overlooked the one machine superior to all those three: the BBC Micro.
It was really hard to buy one in Johannesburg. I found one importer and only one local shop that had software. ZX C64, Atari 800XL much easier to find.
Friend's father had a horror of a PC box. Little did I know...
The ZX Spectrum is definitely the most overrated of all of the 8-bits : it had by far the most horrid (all but monochrome !) graphics of the bunch, and few other redeeming qualities (mainly its price). And yet, there's no shortage of reissues, books, and modern worship around it. Nostalgia is a strange thing, sometimes.
The C64's cult following is much better deserved : in spite of its washed out graphics (it doesn't even have a proper red colour!), it did have hardware sprites, a lot of character and of course the SID: the most exquisite sound chip ever made !
But probably the most underrated and best 8-bit of all was the Amstrad CPC : It had hands down the best graphics of the bunch (Google ZX/C64/CPC screenshots of Operation Wolf, Gryzor, or Stormlord and you'll know what I mean ! Also, check out this recent and jaw-dropping homebrew CPC remake of the Amiga's Pinball Dreams : https://www.youtube.com/watch?v=HVv-oBN6AWA )
The Amstrad was also the only fully standalone one, with its built-in tape deck or disk drive and sharp CRT screen out of the box (the screen also doubled as a PSU, and provided proper RGB connectivity, unlike the mushy Composite output of the C64 or ZX on the home TV)
IIRC, the C64 sold 17 million units, the various ZX variants 5 million, and the Amstrad 3 million. The markets were quite local : the C64 was king in Germany and northern Europe, the CPC was strongest in France (where nobody had ever seen a Speccy ever, and hardly any C64s) and Spain (together with the Spectrum, but not the C64)
Why the British had this strange fascination with the Spectrum and mostly ignored their also-homegrown and vastly superior Amstrad, is a topic of great puzzlement and affliction to me.
And I'm totally not saying that out of spite that the publisher of the gorgeous "Visual Compedium" retro book series, was worried that the Amstrad was "too niche" when I suggested to him that he should do one for the 8 bit with the best graphics (because let's face it, "ZX Spectrum : a Visual Compedium" is kind of an oxymoron). The latest book he had just released was about the Neo Geo. Talk about niche ...