The reason is that CPDLC is still in the test and development stage ( albeit a very advanced stage ) and Maastricht is a test centre for CPDLC.
There was some talk that the Brits/Irish were going to use it to pass on oceanic clearances but I don't know how true that is.
As with everything these days it all comes down to money and how many operators are willing to fit the system and there are still a couple of isssues with it
You might be able to find more information here :-
EUROCONTROL - CPDLC