Question about UWA (Sigma) lenses

This might apply to ther brands but I was looking at Sigma lenses when this occured to me.

I have been thinking seriously about getting the Sigma 10-20 UWA as it would complement the 17-70mm I have now. Then it occured to me that I already know I am going to be working at 10mm or somewhere at these shortest end of the lens so I started thinking that maybe a fixed focal length 10mm prime tht is nice and fast would be an even better solution.
When I looked at the Sigma line I noced 2 things. One was that there is no 10mm prime, 14mm is the shortest and the second thing was that the 2 lenses they do have that are 8mm or 15mm are both fisheye lenses.

I love the idea of the 10-20mm if there is no prime. Is there a prime that is 10 or 12mm and perhaps one that is comparably priced to the Sigma. I would abandon the idea quickly if the answer was to spend more.

Does anyone know of a reason why they would not make a short prime equal to the short end of their own zoom?

I dont know if there is any advantage of having a fast prime UWA lens as most UWA shots I have taken were well over f/8.
I heard the Toki is coming out with a 10-17mm and that will probably compliement your Siggy 17-70 very nice but I believe it is a fisheye.

I just thought it was interesting that you could not get a prime in a 10mm focal length that was not a fisheye. I am not really concerned about fast because these lenses gather so much light to begin with.

go with the 10-20. no such prime exists, and the zooms are so very good stopped down to the Fstops you will use for landscape anyway, that there wouldnt be much of a reason or demand. get the zoom and enjoy