Personally, batteries-included frameworks like Cappuccino scare me off for a few reasons:
1. I have to learn Objective-J. I've been web programming for years and I've become really good at Javascript. I even enjoy it. Other frameworks don't require learning a new language.
2. Time spent learning Cappuccino is only good for programming Cappuccino apps. Time spent in micro frameworks generally translates to other frameworks. Backbone is so bare-metal that it is very easy for Javascript programmer to feel comfortable.
3. Batteries-included frameworks often have "their" way of doing things that are not extensible. This can lead to awkward situations where you need behavior that is just not possible without monkey-patching the hell out of the frameworks and getting yourself stuck on a particular version.
4. I can't speak to Cappuccino, but a similar framework, ExtJS, has (for 4 major versions and nearly 10 years) lacked a build and minification system that didn't suck. This means you must include all dependencies, leading to 1.2MB+(minified!) blobs of JS that can take upwards of 10 seconds or more to load even on fast connections. For business-class apps running on B2B contracts, that might be fine. For public-facing webapps, this is murder.
5. People like micro frameworks because they can use every brand new & shiny piece of JS that shows up on the web without worrying about compatibility. And they can use {HTML5 feature} without worrying about compatibility. I'm sure Cappuccino does a better job than most when it comes to this sort of thing, but what happens when the devs lose interest? Years of "Objective-J" experience go to waste.