Fiveman: Probably because it came right before Jetman, and because it wasn't as dark as Jetman. It's just been lost in the shuffle of 33 different sentai. Jetman I thought was one of the best sentai during the 90s. The pace of the story quickly picks up after episode 10.
Gingaman: A lot of online fans sure love to point out a lot of stupid things from this show. Like their costumes. To me, I find the suits as simple, like Zyuranger's. Gingaman like Fiveman, also suffered from the "not dark enough" syndrome from fans. Gingaman was also the series that got me into the whole sentai franchise. And that's pretty good. Not to mention Gingaman probably has one of the most original themes in sentai: star-beasts! Not to mention had a large cast of very colorful and likable characters.
Abaranger: This one probably suffered the most from the "not dark enough" syndrome. Abaranger is definately one of the best stories to find in Super Sentai. Any fan who watched Abaranger from ep. 1 to 50, including the movie and understood it would know what I'm talking about. Mikoto's story was very well done, as well as Asuka and Mahoro's relationship. Unfortunately, many fans are too busy paying attention to how annoying the goofy scenes are, rather than seeing the good stuff in the show.
I say these three shows have been looked in the wrong way by many fans.