Shannen Doherty doesn't resemble Gene Tierney at all...what a weird proclamation, IMO.

To answer your question, I don't think there's an obvious reason why Tierney isn't as well known as some of her contemporaries. I think it might be because actresses like Gardner, Taylor, Hayworth, Hepburn, and Kelly all had narratives which went beyond their film careers. Further, most of these women are also credited for being style/beauty "icons"--they cut images of femininity and glamour which attract nostalgic fascination to this day. Not to say Tierney isn't worthy of similar adulation, but I guess she didn't have the same "hook" as these actresses. On top of being a great beauty (rivaled only by Hedy Lamarr IMHO), she was also an underrated thespian. I personally think she should be more well-known, but at least her starring role in "Laura" has sustained her place in the Hollywood canon.