[Kamen Rider Lucha]

Pretty sure the answer is toy sales, and it has almost nothing to do with the plot or story.

First, keep in mind that in Japan, the main toy sellers for Sentai are the combining mecha (and, more recently, the collectible gimmicks). The individual hero action figures are generally a secondary seller.

In a market where action figures are already a secondary seller, it seems unlikely that many kids would be interested in buying multiple versions of the full team of hero action figures. But! They might buy a duplicate of one, if his (or her... but hahahaha at that thought) power-up form is cool enough.

Then factor in the costs required to design and produce the molds for a whole line of powered-up sentai heroes. It costs 1/5th the amount to design and manufacture just one powered-up hero.

Add all those factors together, and I think you have your answer.