Same reason as Ashborn I believe. To build up strength of his body to contain the full power of a monarch.
But since baby Suho had a lot of authority, maybe its to induce mental growth by facing countless adversities and challenges and to form relationships with the monarch's descendants

Probably to strengthen his mentality and his power equally because if he had monarch level power with the mindset he had at the beginning of the series he would nothing more than one of those shitty Japanese isekai shounen MC jinwoo really thought this through and also the entire fight with different versions of yourself trials taught MC to be able to fight enemies stronger than him and enemies equivalent to him

Jinwoo didn't intentionally design Suho's system to take away a power. He's just being a parent. Most parents don't want their kids to have to go through what they had to. Jinwoo first sealed away Suho's powers as a toddler because he didn't want his son to be ostracized and possibly develop a warped personality just because he happened to have special powers. He then designed Suho's system to basically be an easy way to get stronger without having to go through all the things Jinwoo had to go through because as a parent he wouldn't want to put his kid through the same harsh experiences he had to go through as much as possible. The class of "Shadow's Heir" is probably the class Jinwoo intended Suho to get since that would put him through the least harsh experiences since the system could then automatically guide him. The class of "Soul Striker" is possibly Jinwoo's intent that maybe his son might not wish to follow in his father's footsteps so the class was basically a second option created from Suho's past actions to be a suitable class in case Suho didn't want to become the heir to the Shadow Monarch. That class only loses the shadow powers because Suho never used them when he was doing the tutorial. The third class of "???" I think is only available because Suho's system is currently glitched and was never something Jinwoo originally intended for his son since he basically loses everything he already has in exchange for getting a completely unknown class that Suho has no way of knowing if it would really be suitable for him or not since the advancement quest made half his powers inaccessible (his shadow powers and his fusion abilities with the Monarch's heirs) so it'd be basically like he's doing the tutorial again so the class he'd get would be similar to Soul Striker but probably more gimped. Also most likely he'd end up having to go through harsh experiences like Jinwoo did when he had the system. However since the system is currently glitched, Suho is essentially picking to forge his own path instead of picking between set paths.

Because the toddler could literally destroy the world on his own.

My guess, it this isn't the first time Suho has gone through this. From the different versions of him, and the different memories he experienced. Jinwoo likely used something similar to the "reset" after Suho either failed, or didn't progress efficiently enough to combat what comes. The version he just fought must have lost in his timeline because he didn't "struggle" enough to progress, like Jinwoo did. So taking away his natural talents to make him earn it through adversity.

Yes,
Elaborate please.

tell me more please

A friendly reminder that Jin-woo was being carved into the ideal vessel for for ashborne, this version is to help sushi grow as he wants, and of course he’s taking all classes, not way he’s getting stiffed after what he went through