Cheap and powerful
Commodore boss Jack Tramiel wanted computers “for the masses, not the classes”. The C64 was cheaper than the competition and handled smooth graphics as well as music. It sold in the millions.
SID music
The SID sound chip could create rich synthesised sounds. Music from C64 games still has fans today, and people keep making remixes of it.
The demoscene
Programmers competed over who could squeeze more effects out of the computer. Many later game developers came out of this “demoscene”. In the West the C64 was what the ZX Spectrum and Didaktik were here at home.