But if there is a way to make Bitcoin stronger NOW, why not do it?
In my opinion, that's the (only) reason:
Whatever quantum resistant algorithm they implement today will either be completely outdated by the time it is relevant
So, even if we used a stronger algorithm today, it wouldn't last long 'til it was also considered unsafe in the long term. I don't know when will it be relevant, but it should definitely take a long time until someone solves the discrete logarithm problem within 10 minutes.
Let alone, it'd make the system less efficient.