some of it such as vo2max has as reason that it's firstbeat licensed (although that may now change as garmin bought firstbeat)
other things such as grade, distance last lap, etc... has as reason that it can be calculated by the developer. Memory is limited for the api as well; just as it is for us.
I also personally think that some data is purposely kept out of the API because it isn't available on all watches (e.g. Grade IIRC). That's just a pet theory of mine, though.
