Sigma Lenses will come for the R mount. I think it is a love hate relationship between Canon and 3rd party lensmakers. On one hand, Canon does not like it that they sell lenses they coul dsell themselfs, on the other hand, they have to "allow" other companies to make lenses for their system, simply because it helps the system getting more interesting (bigger variety of lenses, also special lenses. And just imagine, Sigma (or Tamron, or Tokina) would make lenses for Sony and Nikon, but not for Canon. That would be a reason to switch the system.
I think, Sigma had to wait until they had enough information about the Nikon and Canon mount, especially flangedistance and -diameter cause this affects the optical lens design. The electronic part will be a lot of hard work reengineering it, but basically, they can keep working with the EF system, and just use this functionality at the beginning.