Adobe Photoshop engineers have commented on this over the years.
PNG is not viewed by Adobe as a working format, just an end-product or final-output format.
For example:
http://forums.adobe.com/message/4000022#4000022
PSD files support layers and more metadata than PNG.
PNG is not a working format, but something you export to when you're done with all your edits, similar to JPEG or GIF.
and
http://forums.adobe.com/message/1205501#1205501
Unfortunately, PNG has only minimal (and confused) metadata standards.
You can tag it within an application, but the application doesn't have a standard way of putting the metadata into the file.
There will be more hits if you do a forum search and add Chris Cox as keywords to your PNG metadata search.
A thing to remember is that web sites very often will convert PNGs to JPEGs and do it very badly, beside stripping metadata.
