why is the soundtrack to 'Rainbow Bridge' not available new anywhere
when so much other Jimi Hendrix material is available

I doubt that anyone can answer this with any certainty.

This newgroup post says that "Rainbow Bridge" has not been released on
CD because of producer Alan Douglas:

http://groups.google.com/groups?selm=64b9im%24h9c%40mtinsc04.worldnet.att.net&output=gplain

Here is a quote from former Jimi Hendrix record producer Eddie Kramer:

"I think I mentioned we have tons of material to release, and we want
to take the time and effort to release only the best. If it includes
those two albums, we may. Cry of Love and Rainbow have been released
in essence. To any fan out there: Please be patient. Only the good
stuff will survive."

http://cgi1.usatoday.com/mchat/20010507001/tscript.htm