With Qt/Qt Quick you get an environment that is sort of competitive with where iOS and Android were over a year ago. Meanwhile, iOS and Android have moved on and are accelerating development of their SDKs, and Nokia is falling further & further behind.
Anyway, say you build your app, thinking how great it will be with the huge reach of the Symbian platform. You ship, and discover that (a) people who have a Symbian phone and can afford to buy your apps are just waiting to upgrade to iOS or Android, and so won't invest any time in new apps on their Nokia phones and (b) Symbian is only growing in use in the developing world, where people are very unlikely to buy your app because the cost is a lot more significant.
So you give up, and go back to iOS and Android where the toolsets are nicer and there is more money to be made.