I think its more about how he can judge people, knowing who he can trust and who not to. The crown prince believe in Irene's facade so I'm pretty sure he wouldn't know if his people are betraying him or not.
So he needs a Queen thats good at it!(๑•ㅂ•)و✧
In the original text it says how the crown prince although excels in many things himself he doesn’t know how to surround himself with the right people (like his two faced gf lol) and the 7th prince had the ability to be a good judge of character and put the right people below him. That’s what made him a better candidate
Smooth(〜￣△￣)〜
well there's a fine line between knowing you lack and thus listening and delegating to others versus becoming a figurehead. Certainly it is valuable but that didn't convince me to hand the title of crown prince to another kid yet.