No, GPU is definitely not the issue. The main problem, as Ace already said, is that Cycles allows you to do things that just don’t work with BiDir - such as e.g. ray visibility or changing your shader depending on the incoming direction. In general, for BiDir to work, everything has to be fully symmetric, tracing the same path from the other direction has to produce the same path - but if you have e.g. the path A<->B<->C, where A is diffuse, C is glossy and B is only visible to diffuse rays, tracing left to right would hit B, but tracing right to left wouldn’t.
So, BiDir in Cycles would be possible, but only with a reduced feature set (no Ray Visibility, no Light Path node, probably even no OSL etc.). It might be introduced at some point, but fragmenting the feature set isn’t really that great for usability…