Simple,
Brass is hard to get, VERY EXPENSIVE, and the performance advantage is not all that great over the 338 Lapua Improved versions or 338-378 Wby. At least not enough to justify the extreme cost of getting set up for one. Not sure what you are referring to about it "Killing" all other 338s. That is certainly not the case. Its only marginally higher in performance then the 338 AX or 338 Kahn.
In fact, a 338 wildcat based on a 408 CT case will far out perform the Excalibur and is much less costly to get set up for.
As far as the 358 STA, I agree, its a great round but the 35 cal is about as popular as the 8mm in the US. It just never has or ever will gain any real popularity. Its to bad because its a great round. I built one for my brother based on a Ruger M77 MkII modified to accept a full length Rem 700 mag box. Turned out very well, a very lightweight, HEAVY HITTER. I believe he is using the 280 gr Swift A-Frame and it is a serious performer that will even out perform the great old 375 H&H.
Back to the reason its not more popular, simply because its based on a very unpopular caliber size, nothing more, nothing less.