Because it's hard to make a hybrid app that feels like a native app. It depends on what exactly you are doing, and the state of the art has improved (see the ionic project), but it can be really tough.
Source: I participated in building 3 apps (not all successful, but two made it to the app store/google play) using Cordova 3, and wrote an ebook on the Cordova CLI.
This is a good point... most people working on mobile have an affinity for design and feel they cannot get the same look-and-feel. I think a lot of these frameworks have caught-up to the look-and-feel needed for mobile, and folks should re-consider.
(Ionic creator here). One thing people don't always realize is that many apps are already hybrid. They have a web view and display dynamic content in that web view. I think that meets most definitions of "hybrid."
When it comes to the question of what hybrid apps can or cannot do, Image processing is definitely something they can do. If the JS layer isn't fast or featureful enough for image processing, you can always drop down to native through Cordova plugins or something like ngCordova (http://ngcordova.com/). We always advocate full utilization of the platform, just like we would if you were building a node-webkit app.
I do agree that Hybrid is growing, and more people are finding it a good solution for what they need. That will only continue, and we hope to be helping them for years to come.
I've gotten a few comments[1][2] from people saying Appcelerator compiles to native apps. I understand this, so do Cordova. The point is that you can write your app once, and compile it for many mobile platforms.
Maybe these aren't call Hybrid App Frameworks then? Is there a better term for frameworks like Cordova & Appcelrator?
The result is a highly rated app that's both fast and performant for the end user, while also cost/speed efficient for the business to run and maintain.
Source: I participated in building 3 apps (not all successful, but two made it to the app store/google play) using Cordova 3, and wrote an ebook on the Cordova CLI.