I myself think that the old 50mm formula isn't as much beloved as back in the 90's. Back then, the 50mm lense was the "normal" view of nearly every SLR-Cam. While the digital cameras went up, the full frame was very expensive and until the 5D or the D700 (Nikon) came out... so most of the people went to APS-C.
Today APS-C seems so be the largest
market for camera-makers. So, if you do a brilliant 35mm 1.4 you can sell it to both cameraowners. Giving the usual 50mm look for the APS-C and a (small) wide angle for the FF-Users.
So, in the last years there came up a lot of fantastic 35mm lenses but not tooo much new 50mm lenses, at least not from Canon. Nikon updated just recently, but more pushed because of Sigma or Samyang I think.
In my very humble opinion the 50mm range isn't too desired anymore. For cropping, the 35mm has more reserve (and more DOF) and for potraits you normally choose the more flatten look of a 85mm. So I think the 50mm from Sigma is a result of a good sold 35mm ART. And if the Sigma beats the Canon 50mm f1.4, canon will release a new 50mm f1.8 IS... which is already designed, I think. They will just release it under pressure to save the old one (for the moment) until it hurts