I have worked with both, including Widgets and Quick. I wonder why you think Qtcom is focusing development on Quick, which uses QML (JSON-like syntax) and JavaScript (ES6), while declaring Widgets "feature complete", mostly releasing minor fixes, in other words, keeping it stagnated. The obvious answer for me without getting into Mobile vs Desktop development is: it's easier to get new professionals to use technology as popular as JS (or any other web related stuff), and definitely cheaper for companies to hire said professionals, therefore making it a easier pick for new projects. To summarize the benefits: easier for junior devs to use and because of that overall cheaper/faster deliveries. Note that I'm not saying anything about quality.
Junior developer doesn't mean a developer who just knows JavaScript. Cheaper developers doesn't mean faster or cheaper delivery in the long run. And I don't think brundolf meant cheaper developers.