So an interesting question occurred to me while play the Kingdom Hearts 1.5 hd remix. I realized that after Riku fell into the darkness and the keyblade was passed down to sora, he wasn't able to wield it again until he returned to light. But Xehanort gave into darkness and can still wield his keyblade. So my question is how come Riku can't wield his keyblade when he gives into darkness but Xehanort can?

Because the Keyblade he was destined to wield was THE Keyblade of Light. Although, you could also argue that Mickey shouldn't be able to wield the Kingdom Key D in that case.

 

I think it was just a huge coincidence. At exactly the right moment, Riku plunged into darkness just when the Keyblade was going to become his. In his place was Sora, who was the next best thing. Maleficent then gave Riku Soul Eater and he wasn't aware he was supposed to wield the Keyblade (at the time) so nothing changed until he summoned it himself.

Edited July 12, 201411 yr by Kaweebo

Riku was lost to darkness so it weakened his heart . Once he learned to control his darkness his heart was strong enough to wield again . It's not darkness that causes the loss of the keyblade . It's being consumed and controlled by it that weakened his heart . This is why MX can wield with so much darkness. He has learned to use and control it and therefore his heart isn't weakened by it .

Nomura said so

The darkness weakened Riku's heart, after he got used to the darkness and resolved to not be consumed by it anymore is when he got his new keyblade.

After falling he hit his head and forgot for a bit

His heart was saddened by his parents' divorce and the revelation that his girlfriend had an affair with Ansem.

What FL said. It wasn't the darkness, it was his weak heart that caused him to lose it to Sora. To wield the Keyblade one needs a strong heart, strong enough to stand against or subjugate the darkness.

Edited July 12, 201411 yr by Isamu_Kuno

Riku was lost to darkness so it weakened his heart . Once he learned to control his darkness his heart was strong enough to wield again . It's not darkness that causes the loss of the keyblade . It's being consumed and controlled by it that weakened his heart . This is why MX can wield with so much darkness. He has learned to use and control it and therefore his heart isn't weakened by it .

Okay. That makes sense!

His heart became incompatible with the Keyblade. At the time his heart was weak and easily manipulated by the Darkness, and a person with a weak heart cannot weild the Keyblade.........I suppose.

Edited July 27, 201411 yr by Pandy Monium

Riku's Heart was weakened by his over-exposure to Darkness and then the Keyblade left him for Sora. (Wait...)

When fell to darkness his heart was weakened for time to the point where he wasn't able to use the keyblade for a while.