If Krishna really wants us to be happy, why doesn't He let us eternally and completely control and enjoy maya?

The reason is precisely because He wants us to be truly happy. He's the supreme enjoyer—that's His constitutional position—and we're His inseparable parts. That can't be changed. Constitutional means essential and unchangeable. We're meant to enjoy in cooperation with Him, but when we choose to behave differently, we're choosing to not be ourselves.

Krishna kindly provides us with a false identity—so we can think we're our bodies—and a virtual reality in which to try to have fun. But since we're spiritual by nature, we can never fully enjoy or be truly ourselves in a non-spiritual environment. Krishna is fully aware of this, and arranges for us to go through a period of attitude adjustment—trying to enjoy in the material world but being frustrated—until we finally realize the truth about ourselves, and begin the process of inquiring about our real purpose for existence. Then we can begin the process of re-entry into the spiritual world.

While we're here, we can imagine we're controllers and enjoyers, to some degree, but our ability to do so is limited and temporary. We can't enjoy completely—there's always some fly in the ointment—or forever, because death keeps getting in the way.

Theoretically, since He's all-powerful, He could make us equal to Himself in all respects, and give us the ability to enjoy and control. But because He likes loving relationships, He prefers our love more than anything else. He is the ultimate authority on enjoyment, and if He thought we could really be happier by eternally living in illusion, He would arrange for that. But He knows better, and that's why our ability to enjoy and control maya is temporary, and our real enjoyment comes when we work on developing our loving relationship with Krishna Himself. That's just the way it is.