Presumably because sales were too low to justify production. Insufficient demand is usually the reason for product discontinuation.
.40 S&W is currently in sharp decline as LE agencies are dumping it for 9mm & civilians are following suit. I think it's less popular than .380 ACP in ammo sales right now.
.357 SIG has basically been dead on the vine for a while, if it ever was really successful.
Federal has just 1 load* (an FMJ!), Win has a veritable smorgasbord of 4 whole loads* (3 fewer than they have in 10mm), and Rem & Hornady have a whopping 2 loads each*.
* Catalogued, what they are actually making or will ever make again may or may not be the same thing.
I'm not saying it's not a decent round, just that it never got that popular, and almost nobody is currently buying it.