When most of the frameworks focused on bringing loads of fancy feature and UI using some library or other Zepto.js just focused on mobile WebKit browsers and tried to unleash its capabilities by cutting down the extra bit of codes written to address the cross browser issues. It absolutely makes sense. When the browser is capable of supporting rich features why not use it and reduce js code. Thats what everyone needs to do.
The size of the Zepto.js is just 2k. What more you can ask for to use in a mobile environment. The good news is , Zepto supports jQuery-like syntax and chaining. People familiar with jquery can immediately start experimenting zeptos.