Cost, for one thing.
It is simpler to fit a fighter such as an F-16 with a system such as the Sargent Fletcher 370-Gallon Aerial Refueling Tank/System rather than add the weight, drag and complexity of a boom to an existing tanker.
The US has plenty of boom equipped aircraft for its large receivers; the RAAF's KC-30A has a boom system to meet the requirements of its national large aircraft receivers.
Whilst large offload rates and quantities might be more suitable for large aircraft, the demands of modern fighter aircraft formations are better met with the multi-point hose and drogue system. Not that you'll get the USAF admitting that.....
