Why can't I run the WWDC app?
I can't speak for Apple, but this is likely an intentional decision to walk away from the server side / api calls for the older version of the WWDC app.

One of the core messages for iOS 11 is that all 32 bit platforms and apps are done for developers, and Apple making a hard stop of functionality (programming in a message telling you your app is too old). Programming in a kill switch could be seen as reinforcing that experience for developers. 

The temporary good news is despite this move, you can still watch WWDC 2017 content on iOS 9 in Safari. The news on 64 bit only requirements was covered in the Platforms State of the Union keynote (2017 session 102) I believe.