I suspect it's to do with ramp space. I've also heard that there may be some issues about how close it is to the runway.
I suppose an extension to the new terminal could be relatively easily built over the car park in front of the old terminal.
That said, this costs money and that's something that Cork Airport doesn't have. Even if it was available a parallel taxiway or runway extension would be a better way to spend it.
Are there any problems with asbestos in the old part of the old terminal?