[ShiShiLeo]

Not exactly sure how much it applies to Kamen Rider, but for Gundam gashapon toys, the designers are allotted a budget for each wave of figures. A really nice figure in a wave means the wave is also gonna get a goon or repaint.

And sometimes a line of toys is not profitable at all (due to a combination of costs, sales, and associated popularity), but the line will continue as a legacy sometimes at a cost to Bandai. The high quality SD Gundam Gashapon Senshi Forte line being an example. The SD gashapon were such a crucial part of Gundam's early popularity that it would be shame for the line to end today.

And when a line gets to that point, the designers often make figures of stuff they personally admire or know for sure will sell because its older. In Kamen Rider's case, the Showa figures which appeal to older buyers uninterested or lacking in time for stickers and assembly. Also, the Heisei phase 1 riders are much less complicated than the phase 2 ones.

Source: they often talk about this aspect on the gashapon blog https://gashapon.jp/blog/?p=7758

That being said, all the SHODO I have are super impressive. More articulate than SODO, better detailed, and great paint applications for their size (significantly smaller than SODO). My Ichigo figure even has drop-down hips to allow him to ride his bike! The plastic is altogether different from SODO, being much softer and most often sublimated rather than molded into the color.

Unfortunately, SHODO are even harder to find than SODO, otherwise I'd probably be collecting those!