Think Scotroutes summed it up, a fair bit of Fatbike kit is not covered by so many standards as regular MTB is.
Cranks, hubs, and a fair bit of other stuff if building from scratch needs careful selection. Front hubs are 135mm, rears, 2 sizes, 170 or 190 depending on tyre width, I think Surley Pugs are different again…. then making sure that there is clearance for the gears to get full range around the tyres. Front mechs often need spacing out and are different too.. Then forks, adding the current Bluto suss fork will hit the downtube on some frames if rotated far enough..
There are also carbon frames and rims from China if you want to go that direction…
Don’t get me wrong I love building from a frame upwards but the bits can be sourced individually but you need to make sure its all going to fit together. Plus as a package it will costa fair bit less and you will be riding sooner.
Its not a world of pain and dont let me put you off either, its just different getting Fatbike stuff together for a self build. Perhaps a good frame with a Lefty fork, its possible but needs bringing together.
Spesh, Pugsly, Mukluk, 9:Zero:7, Salsa etc etc all make Fatbikes with good kit and increasingly, a suspension fork. Given that a Bluto fork is over £400 alone.. adding one to a rigid bike is expensive. Almost better to buy a bike with a Bluto and add a rigid fork if you want…. but suss (or Full Suss Fat), does seem to make sense. (I have suspension for summer trails and rigid for winter / snow)
HTH