His heart may not have been strong enough to wield one when he was Xemnas. The only times we see an incarnation of Xehanort wielding the keyblade are when he is whole: as YX, AX, and MX. If he had been seeding the 6 apprentices before splitting himself, it might have weakened his heart too much.
We also don't know everything about the nature of the different Keyblades. MX's keyblade is a dark keyblade, right? Maybe it requires more power to draw upon than a light keyblade or some other different requirement. And the keyblade doesn't just need a strong heart to wield it, but some other intangibles, or everyone with a strong enough heart would have one. Those intangibles could have been split between Xemnas and Ansem.
We also don't know everything about the nature of the different Keyblades. MX's keyblade is a dark keyblade, right? Maybe it requires more power to draw upon than a light keyblade or some other different requirement. And the keyblade doesn't just need a strong heart to wield it, but some other intangibles, or everyone with a strong enough heart would have one. Those intangibles could have been split between Xemnas and Ansem.