Why aren't the fields of the URI class available in PCL?

Question

I noticed recently that the fields on the System.Uri class are not available in a PCL.

Specifically, these fields:

Uri.SchemeDelimiter
Uri.UriSchemeFile
Uri.UriSchemeFtp
Uri.UriSchemeGopher
Uri.UriSchemeHttp
Uri.UriSchemeHttps
Uri.UriSchemeMailto
Uri.UriSchemeNetPipe
Uri.UriSchemeNetTcp
Uri.UriSchemeNews
Uri.UriSchemeNntp

as well as others.

Is there a technical/pragmatic reason why these wouldn't be supported in a PCL? I see in the reference source that these fields (except Uri.SchemeDelimiter) come from internal static fields on System.UriParser. I then noticed that UriParser is not PCL-compatible.

Is that the reason?