AtG, the US, Australia, Canada, New Zealand, France and others also use their MPA for overland ISTAR, your statement is simply wrong.
It just so happens that an airframe with long range, endurance, extra capacity (space, electricity, crew positions) and bunch of sensor operators is quite good at being adapted at short notice and low technical risk to do lots of other rolls. The term MPA should be put in the history books, MMA (Multi Mission Aircraft) would be a much better description of what they are.