It was the best-selling single model of home computer for a very long time (relatively speaking in a very fast-moving field) in an era where most new models ditched compatibility, and in some countries it totally dominated.

Where I grew up (Norway) you rarely if ever saw Apple's until the Mac, and only the occasional other brand like Amstrad or Spectrum.

In my primary school classes, almost everyone who had a home computer had a Commodore 64. As a result, it was easy to get (pirated) games.

The network effect was strong - having a different computer meant you might have nobody nearby to swap games with. I knew one person - vaguely - with an Amstrad, and one person I knew of at my school had a Texas Instruments machine, and one with a Spectrum, but there were half a dozen kids in my home room alone with Commodore 64's.

It was much more tribal for that reason. If you had a Commodore 64 or Amiga, chances were Atari was "the enemy" even when Jack Tramiel (who founded Commodore) was kicked out of Commodore and bought Atari, and Spectrum's were just laughed at.

Mac and PC's were seen as boring business computers.