From http://sunrise-product.tumblr.com/post/47802736454/why-doesn...
Why doesn’t Sunrise support iCloud/Exchange yet?
Every other calendar app seems to do it, what’s taking Sunrise so long?
To explain let’s breakdown a calendar app into two parts, the presentation layer and the storage layer. In the stock iOS Calendar app, Calendar is the presentation layer and EventKit is the storage layer, both developed by Apple. Other calendar apps in the App Store also follow this two layer design but only redevelop the presentation layer while continuing to use Apple’s EventKit storage layer. In other words, these apps quite literally take your data and dress it up in a different color. What we do at Sunrise is redevelop both the presentation layer and the storage layer.
You guys are crazy, why bother reinventing EventKit?
The storage layer is responsible for keeping your data in a structured format. If we were to use Apple’s EventKit storage layer, we would have no control over what can be stored and how to store it.
Okay, so what’s wrong with not having control over the storage layer?
The storage layer needs to be revolutionized. Why can’t my calendar store anything besides text? What if I wanted to attach a picture to an event? Better yet, why not a video? What about other rich media that I would want to be included?
Herein lies the problem, the current storage layer isn’t capable of storing anything other than basic data. Not only that, it only stores certain kinds of basic data: titles, descriptions, dates, etc. We want to break these barriers and let you store anything!
Revolutionary features are coming to your calendar. We’re working as fast as we can.