This is not just an arbitrary requirement. You can test on a physical device running iOS 6.1, or you can run Xcode 5.1.1 in OS X Mavericks to test your project in the older simulator.
You can still build your apps to support older iOS versions by setting an older deployment target, but you will not be able to test them on a simulated device running on OS X Yosemite. IOS Simulator runtimes older than iOS 7.1 will not work on OS X Yosemite.
