but I still don't understand why it's not more popular.
^with regards to 10mm.
I think it's a simple answer. The popular rounds (especially in service sized pistols) have all had the benefit of
widespread police or military use. The 10mm's service career was just a scattered footnote in history, comparable to the .357 Sig.
There are, IMO, two distinct levels of semi-auto cartridge popularity, with one half-way in between. You've got 9mm, .40 S&W and .45 on the top tier for obvious reasons we don't even need to discuss.
You've got all the "standardized" rounds in the lower tier that simply won't ever make it with the big three. 10mm, .357 Sig, .38 Super, .25/.32 ACP, 9x18 Mak, .45 GAP, .50AE and some others with a bit of argument.
In that slim slot right between would be, IMO, the .380 Auto.
The .380 Auto was solidly in the same second-level group just 10-15 years ago before the concealed carry wave that swept this nation and saw a public demand for smallish, concealable pistols. But it has had a meteoric rise due specifically to tiny carry guns. It's a pretty interesting feat for a cartridge that's been around as long as it has.