They did explain the reason. Excalibur can match soul wavelengths with anyone, grants wings of light, the ability to teleport and can cut the heavens in half with one swing. But there isn't anyone who can match him as a person.
No matter how powerful he is, he is one giant raging faggot. And no one can stand him long enough to wield him. Plus, he has all that bullshit paperwork to go through just to become his Meister/Technician. Fuck that shit, I'll pass.