Because the old version have a not so good design
Lots of the apks from the old version don´t work with marshmallow
The old verison wasn´t ok to upload to the PlayStore
The old version have no note about the original developer or source from the app
What I guarantee for every app that is in PortedApp?
That every app that is listed worked without any problems on my S5 and Nexus 5 both with Android 6.0 (Marshmallow) when I tested it out