why isn't delivery a ground frequency?
We trialed GMC back in 2003/4 and again in 2009ish using different configurations but the physical layout of the airfield didn't lend itself to having GMC as a stand alone frequency. After consultation with the based airlines and BA, it was decided that the better option was Delivery. We can give clearances up to 30 mins before the filed departure time and when Delivery is open during busier periods, that position also deals with CTOTs, ready messages, and in the case of yesterday, constantly updating FPLs. This relieves the burden on the other ATCO who can then concentrate wholly on moving traffic.
And to answer the unasked question, we only planned to open it when it's needed thus why it's normally only open first thing each day.