I attempted to watch a couple episodes of Kirameiger last week & I really think that, for the most part, Sentai just isn't my cup of tea. I think Kirameiger has got a great, irreverent sense of humor.
In the end, (and I'm loathe to say this, fearing Toei will want to dumb down Rider to save Sentai, assuming there is a real problem) I think the reason is simple. Rider feels more adult than Sentai & kids hate feeling like they're a little kid, so even they're embarrassed to watch it or admit that they like it for long.
Several years ago, I got my 3 year old goddaughter into Lost Galaxy, which she almost immediately fell in love with, got super invested & wanted to start watching Dino Charge on TV to. Within a day, she was running around the room telling people what Megazords were, she fell over laughing every time Trakeena came on screen &... apparently learned the word explode for the first time. But, her dad didn't want to sit around watching it all day, and kept openly disparaging it every time he caught her watching it & a week later, she went from loving it to completely hating it & having an extremely difficult time taking it seriously. I couldn't even get her to watch Time Force.
Anyway, wierd tangent over-- despite how I feel, I don't want Sentai to change into something it's not, though. We already have a Kamen Rider. But, so far, I've only seen Dairanger & Kakuranger in their entirety, plus a few episodes here & there of Gekiranger, Goseiger, Gobusters & Kirameiger. I only really liked Kakuranger. The rest just shove a lot of randomness in your face right off the bat that you either go with or you don't, the characters are disturbingly happy (and apparently, no one liked the result the last time they weren't-- Gobusters) & there usually isn't much plot to fall back on. But, I was rolling with the Rugby Maskman's evil plan to force random people to play rugby against their wills with an exploding ball, though. Also the monsters growing through an Evil Reimbursement Program.