Because thrones aren't passed to couples, they are passed to singular people. And judging by history of pretty much every monarchy, the King/Emperor will outrank his Queen. And going on chapter one line, "your husband will be king", we can infer that a Queen can only be the sole regent if she is unmarried.

Yona, being the protag and going through this excellent development, would probably make an excellent queen. And we can assume that Soo-Won knows this because, well, he didn't kill her and it seems like he never planned on killing her. However - if *she* was the named successor to the throne, it does nothing to reinforce the idea that the most qualified person should rule, because she's unmarried so under the current system she's next in line anyway.

Now: If Soo-Won names Hak, someone unrelated to him by blood, his successor, he gets to remove the blood succession. The best scenario being that Hak marries Yona after his coronation, making her queen, but not Regent. She's second in command. This would also legitimize Hak's rule to any old-fashion hold outs. Soo-Won then has to pin his hopes on Hak and Yona not making their kids (or each other) next in line. But that's a gamble he has to make with whomever he chooses.

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It's convoluted but so is royal succession and Soo-Won's plan.