Games. Nintendo lost Square to Sony, and Dragon Quest from Enix kinda petered out and eventually shipped on PS1. The effectively had a huge catalog of RPGs that n64 was sorely lacking. There was also a comparative lack of 'mature' themed games. And the catalog was smaller, about 5x so. Even Zelda was not a launch title. Probably because the cards were substantially more expensive than CDs.

There was also a supply issue early on in the n64's lifecycle, I seem to recall it being just shy of Tickle-Me-Elmo hard to find.

The ps1 was also easier to pirate, double edged sword though it be.