the PPAPI is very closely tied to chrome's inner workings and is extremely complicated to implement as, compared to the old plugin api's, it doesn't allow native code
any access to the local system. So it needs to provide plugins with all the possible hooks they will ever need.
Check https://developer.chrome.com/native-client/c-api for a list of currently supported features.
For other browsers to support PPAPI, they'd have to implement all of this, which, btw, also is a moving target that moves forward in lockstep with chrome releases.