Why isn't the default TCL URI package available in iRules?

For lack of a better answer I suppose, the original TMOS authors had specific reasons for the version and capabilities of TCL that was included, and the initial "locked down" posture has certainly been loosened in the most recent releases (ie. proc, sideband, etc.). But I can't honestly say that I've run across any problem that the provided HTTP and string functions cannot do that the aforementioned base URI library can. I would recommend opening a support case to get a more definitive answer to your question.