Because it could weaken NATO at a time when the USA has an unpredictable president and the Russian Federation are still in the Crime under a dubious pretext.
And us leaving the EU will not?
I still can't quite compute why it is apparently good for EU member states to commit to participating more in NATO, yet bad for them to be thinking of organising their own defence outwith of NATO (especially in light of Trump's comments). TBH I wouldn't be at all surprised if Trump really ratchets up the rhetoric on NATO given his thoughts are now turning to 2020, also the Asia pivot.