Sigma stopped all Pentax support the moment the K-1 came out. Since that very moment, they just sold off old stock, but no new lenses were made.
A stupid and regrettable decision, since Sigma was the only lens maker that made lenses with optical lens stabilizer that work with the K-Mount.
With long focal lengths that's something you'd want to have, since the IBIS does not stabilize the viewfinder image.
And Pentax cameras do support optically stabilized lenses since the K10D.
As a reason "low revenue" was given, but that was created by Sigma themselves. Until the K-1 came out, they had several lenses in their portfolio, that were of relatively little use for Pentax photographers, as for instance the 12-24. On APS-C, Sigma had two wider lenses, the 10-20 and the 8-16, both in K-Mount. The K-1 is the first Pentax since the days of film that could use the 12-24's potential, because that's a full-frame lens. In 2016, a new version of that lens was released - no K-Mount.
I had to get a used version of the 12-24, couldn't buy a new one since there wasn't any. And Pentax' 15-30 quite obviously is no alternative if ultra wide angle is what you're after.
Sigma never pointed out that their use of optical stabilization on the K-Mount was something noteworthy; there are lots of photographers around who don't even know that optical stabilization does work with Pentax bodies. And it gives users the choice of both systems, which I deem the best possible solution.
This is utterly stupid and very bad marketing. Yes, Sigma did react in a very customer-oriented way with the firmware and mount upgrade program for several lenses that created mechanical and software problems with the K-1, but instead of building on this, they just ignore customers.
I own and use several Sigma lenses, because there are no alternatives made by Pentax and because they're economically viable. The aforementioned 12-24, and on the other end of the focal range, the "Bigma", the 50-500.
> Just out of curiosity, how many of the participants on this thread have purchased a Sigma product for Pentax, new, in the last five years. Don't be bashful.
I've got the 12-24 used, since they didn't sell it new anymore. It still has an aperture ring.
The 50-500 I've bought new, getting one of the last copies made.
Back in APS-C days, beginning in 2008, I've bought the following lenses, all new:
8-16, 10-20, 17-70(OS), 70-200/2.8, 70-300/3.5-5.6(OS) (not in that chronological order, though)
Of these lenses the 70-200 and the 70-300 can be used on the K-1, the 70-200 unfortunately is a bit soft, it presumably needs some "recentering" or somewhat.
Oh, and the "walk-around" lens I curretly use most of the time is a 24-70, also made by Sigma. That one I bought used, out of a whim for less than 100 EUR, but it works like a charm (though it doesn't inspire trust due to it's plastic mount).
So, at least I do qualify as a "Sigma-on-Pentax" user.