I'm sorry, but just, what?
It's based on the entire concept of a hero who gains a suit and with it powers. And the only manga version of a Sentai series I know of is the Jetman one, and that's an epilogue.
And it's mostly a matter of simplifying it- going with "Power Ranger" flies against the standard class naming scheme of a single word, and could get confused with the actual Ranger class.
So I chose Sentai for the name, in addition to it being a legacy name, for the unambiguoisity of it. And because it sounds cooler.