One of the European contributions to the mission is the launch vehicle.
IIRC also at one point the project was in a big trouble due to cost overruns. Europe joining the effort by providing the launcher, one of the instruments and part of the project team saved the whole project from cancelation.
ESA have the best rocket that can fit it in
NASA, like most US companies, outsourced flying things to space to other entities, since flying things to space it's not it's core competency anymore.