You seem to imply that AirPlay and DLNA are the same thing, and that Apple is taking a proprietary standard and foisting it on people over an already existing open one. This belief just shows a complete failure to understand how either technology works. Anyone who has played with both can immediately see the difference.
First of all, AirPlay is a push technology. You push audio or/and video from a device to another. Typically an iPhone/iTunes to an AppleTV or Airport Express
DLNA is the other way round. It lets a device advertise what content is available and a client will pull from it.
While the DLNA has given lip service to adding push functionality, essentially zero practical effect has come of it. The advantages to push are many:
• AirPlay can be used to get synchronised playback from many different units. Multiple DLNA can playback the same file however, they may not be in sync. In fact, there is very little chance they will be.
• Want to stream internet content? Good luck with DLNA!
• Using push technology allows Airplay mirroring. There is no such equivalent for DLNA.
• Since DLNA simply streams the files, it is limited to the end device's codecs. It is also unable to stream content, such as window contents, or, as mentioned, internet streams.
In addition to these advantages of push, because it uses mDNS zeroconf, Airplay also allows MUCH easier set up and device control. DLNA doesn't give you quite the control over endpoint devices the same way that Airplay does.