If I recall right, they've had a couple of state contracts for .357SIG M&P, but overall the sales of the caliber are much lower than the 9/.40/.45 guns.
If it's not a popular selling caliber for either commercial or LE/Gov sales, it would make sense to drop it from the commercial catalog and just make a run of the guns if a LE/Gov agency were to place an order. Focus on the faster selling caliber models.
The .357SIG is usually described (in armorer classes) as being harder on guns than the others, too. Then, there's the ammunition cost compared to other calibers (which can vary regionally, and depending on whether there's a "contract pricing" source for LE procurement).
Back some years ago (7-9?) I asked a Glock rep how the .357 guns were selling. He told me that he hadn't seen any figures (as a rep), but that when he'd been curious and asked someone in Smyrna about the caliber, he'd been told that Glock made 1 gun chambered in .357 Auto for about every 2,500 other guns chambered in other calibers (he didn't know if that meant in each separate caliber, or just all other calibers, combined).