Technically, the SatComm equipment is most of the time not installed by the manufacturer, but by some 3rd party.
If the manufacturer would have to install it, they typically want to do some more extensive testing as it's then them who are responsible for warranty etc.

Depending on the surrounding circumstances after delivery (e.g. leased aircraft vs. own aircraft) the airline sometimes also might have to wait some time because of warranty and other aspects.
Another reason for a delay in installing the SatComm equipment is usually the availability of the installing 3rd party, the availability of a suitable hangar or the availability of the SatComm equipment itself or, as already mentioned, the airline which cannot affort AOG (Aircraft on Ground) for installing the equipment (which can, depending on the involved solution, take a couple of days).