Well, there's two sides to the process.

Some of the items in devel are there because they are 'half made' - the dev worked on it, got it vaguely functional, though not really very polished or optified, and then got distracted by some other project. On that basis, things can stay in devel for a long time.

On the other side, items get promoted to testing, but don't get enough interest from testers: an item in testing requires good testing, feedback and a certain number of positive votes to progress into extras. Without willing testers able to give good feedback, apps can get stuck there.

I believe there's also a 'wait time' between apps achieving their required karma and being promoted to extras, in case some strange previously undiscovered bug might turn up.

If people are keen to see more apps, taking a bit of risk on a few apps in testing is a way to contribute to the process.