I would like Yesung to release a solo album. Do Ты know why he hasn't yet?

Because he "belongs" to SM. It's quite a widely known fact that SM likes to mistreat или ignore their artists. :/ They're big on subgroups, but I don't they've had any member from any group release a solo album. Not to my knowledge, they haven't, at least.
I, personally, strongly agree with you. He's hardworking and his vocal skills are top-notch. He definitely deserves it. I hope when he gets back from the army they arrange for one to be released.