From the Kotaku website, with extracts here, Highmoon (who are doing the Fall of Cybertron game this year), have come out to justify why there is no PC version.
Short answer - their limited expertise and resources would have ended up with a poor version that would have created more criticism than not having one at all.

It's not going to appease anyone though, as they are effectively prioritising certain versions to make them better products, expecting to sell more of 2 great products than 3 mediocre products.