Qt was originally a proprietary product, and it has long-term proprietary licensees who signed up in the expectation that they would get the code with no copyleft.
The choice when they took it open source would have been (1) cut off the existing proprietary licensees from new versions by using copyleft for everyone, (2) go with a non-copyleft license and lose at least some of the license revenue (probably most of it) or (3) require outside contributions to use a CLA that allows their work to go proprietary.
So Qt probably did the best it could considering the existing customer base, even if that meant losing some potential contributors. The situation was different from a green field project with no licensees.
Existing proprietary licensees at Qt
The choice when they took it open source would have been (1) cut off the existing proprietary licensees from new versions by using copyleft for everyone, (2) go with a non-copyleft license and lose at least some of the license revenue (probably most of it) or (3) require outside contributions to use a CLA that allows their work to go proprietary.
So Qt probably did the best it could considering the existing customer base, even if that meant losing some potential contributors. The situation was different from a green field project with no licensees.