From what I understand, it is due to the programming specifics of the training algorithms, primarily being focused on exploiting certain registers and architecture features specific to Fermi. The code actually got updated from GTX280 series to GTX580 series IIRC, so it is likely that it will be updated again at some point by a motivated researcher or group. I suspect there simply isn't a need to update right now for most labs (though I suspect TITAN / TITAN LE / TITAN II may change that). Also, Alex Krizhevsky now works for Google :), so someone else may need to do the updating.

http://www.wired.com/wiredenterprise/2013/03/google_hinton/

You can check out the code here - it is really good IMO.

http://code.google.com/p/cuda-convnet/