A huge advantage of Qt that it is known by many developers and used by many people on many different platforms. It is true that it is big (it is not a GUI toolkit but a platform abstraction layer) and its desktop widgets are in maintenance mode (we don’t see much feature development, and bugs are fixed at about the same rate as they are introduced).
Unfortunately, recent aggressive revenue-focused behavior of Qt Company makes its future somewhat uncertain. It would be nice to find an alternative, but I don’t think any of the other GUI toolkits come even close.