The Reminders and Events panels both use EventKit which only added reminders support in iOS 6. Also the high adoption rate of iOS 6 makes it not worth while to support older versions.

I've spent many hours on other applications, mostly Mac, where the time spent making something backwards compatible was never really worth it. Example: I worked on getting fancy animations in an app that had to support Panther, which didn't even have NSAnimation support. However at this time Leopard had just came out and had all the new Core Animation hotness. Needless to say I would have saved countless hours if I didn't try back porting new features to older OSes no longer supported by Apple.