There's a lot of compounding factors impacting N64 emulation: lack of developer interest, relative system complexity vs. 16-bit systems (which impacts developer interest), smaller game library vs other consoles, unique controller and peripherals, smaller body of documentation, more limited reverse engineering efforts, the GPU is both complex and bespoke, truly accurate emulation would require more hardware power than we have in PCs today, harder to dump games vs. PS1 CDs, things like that. Plus the early N64 emulators were pretty cliquey and weird and are (mostly) still closed-source, so there isn't as large a community sharing knowledge and building on each other's work. Everyone who wants to build an emulator has to start mostly from scratch.

Having a MIPS CPU isn't that big a hurdle compared to everything else. That said, all of the points I've brought up are currently improving. There's probably more developer interest in the N64 now than at any time in the past two decades. The next release of the Linux kernel (5.12, due out in April) will include support for booting on the N64 and Near, the bsnes/higan developer, has gotten a decent, if somewhat incomplete and buggy, N64 emulator up and running in a handful of months and rolled it into their pet emulator project ares. It may eventually make it into higan if it shapes up well enough. Things are finally looking up for N64 emulation.