I think they just took advantage of having stumbled upon an actor who had bucketloads of screen-charisma. Having said that, while he does shine he's not the only highly developed character. I just think that the actor left more of an impression on the audience, and so the others are overlooked.
In many ways, Ryu is a much more interesting character. A soldier suffering from psychological trauma, forced to go into battle with the very enemy that caused it in the first place, and having to lead a team of untrained, unmotivated rookies while simultaniously having to deal with his own worsening mental health. By comparison, Gai is just a card shark, and not a very good one either (I can't recall a single time he was shown cheating without getting caught). OK, so he falls for a rich girl, tried to drag himself up to earn her love, and despite not winning her heart realises that he's become a better person. Yes, it's better than a lot of sentai characters storylines, but for me it isn't on the same level as Ryu's story.
[HIDE]On a somewhat random tangent, I find it interesting that on reflection, despite Gai being the returning character for Gokaiger's Jetman tribute the actual storyline has Marvellous going through an experience that more closely mirrors Ryu's, albeit on a smaller scale. Anyway, random "bloody obvious if you'd actually seen the show they're doing a tribute to" observations aside...[/HIDE]The bit that spices both storylines up is the way they used Kaori as a catalyst for them both. She gives Gai a goal to attain, while at the same time constantly bursting his bubble by actually being the classy rich person Gai tries to project the image of being. Her affection for Ryu causes her to become something of a walking trigger to remind him of his trauma. While she inspires Gai to become more of a hero, she's actually preventing Ryu from being the hero he once was. Kaori also provides a point of friction for conflict between the two characters, as the basic leader/rebel dynamic isn't enough to sustain a rivalry for an entire series (although personally I found that once Gai accepted his second-in-command role he became a lot less interesting).
The bad guys in Jetman also get a lot of development. Tran/Tranza was the nearest to a typical Sentai villain. The others all have their own detailed storylines. As much as fans discuss the Ryu-Kaori-Gai love triangle, they seem to forget about how Ryu also gets sucked into a love square(?) with Radiguet and Grey for Maria's affections (in which only Grey actually seems to pursue Maria out of genuine affection rather than alterior motive, but is doomed to rejection due to his robotic nature). I'm not sure what the OP meant by Grey having a rivalry with Gai though. Their final battle seemed to me to stem more from them both being of equal status (second-in-command) than anything else.
It is true that Ako and Raita (and Chief Odagiri) seem to lose out by comparison. But there are many sentai seasons where a couple of the main five didn't get much development, so Jetman is hardly unique in that respect. It only seems odd because they managed to pack so much in for everyone else.
