I believe Den-O's ratings were fairly low but it sold an utter shitton of toys. That and the most popular guys are guys in rubber suits with VAs, so it's cheap to make films based around them.
I believe Ryuki was originally planned as a Metal Heroes show, which makes sense seeing as it's magic/science armours rather than cyborg/mutant/mystic transformations. It got good ratings and toys so they continued. Kabuto killed it off by having the secondary riders' merch not sell so well. Which is why we now have form changers and maybe 1 secondary rider.
I plan to watch Decade only when I've seen the other ones before it. This means I'm gonna have to get more than 10 eps into Den-O (urgh), watch Blade (likely raw as the current subs are pretty ropey), Hibiki (I have heard bad things about post episode 30) and Kiva (Twilight with stuff being kicked and time travel).
IB: you seen the currently airing Sentai, it seems to be cribbing a bit from Kamen Rider with the powered modes.