I can't tell about the rest of world, but for sure in Italy it's easy to understand.
- Better marketing: lots of ads during prime time in most popular evening shows.
- Better distribution: PS games were avaliable in almost all small to large computer and electronics shops, while N64 was distributed by a local toy company and its avaliablilty was much more limited and restricted to toy shops
- The price difference for games was insane (due to both cartridges and intermediation costs I think), I remember some new N64 game could cost up to 3 times a new PS1 game.
- Language barrier: the PS1 was the first console with full localization on all titles, when I was a kid it would have been impossible for me to play a game like FFVII or MGS in english.
- Of course 3rd party support
- The value of having a CD player. That could seem irrelevant but it was a big deal at the time.