Ahhh... there are a number of reasons here but mainly:
Fragmentation
The two big current trends are "pod
mods" and multi-drop-in-coil high powered "cloud chasers".
The high power and vapor output folks tend to vape at ~3-6mg. (or maybe less) The pod
mod users generally use nic salts at 35-50mg.
I drop in on my local vape shop from time to time and last time I was there all they had was a whole bunch of different pod mods and a handful of mods/atomizers that were high power.
There was NOTHING that had a replaceable battery and NO atomizers where the user needed to wind their own coil.
They had a wall of different types of
juice however as mentioned, ~3-6mg free base and ~35-50mg nic salts were the only choices.
g.