Xehanort did not give up on Riku because his heart was not "full" of darkness. While Riku's heart has more darkness than the average heart it still also harbors a strong light. This light was which caught Terra's interest to give Riku the ceremony after all.
The reason Xehanort gave up on Riku is because he developed a strong resistance to darkness. Meaning that Xehanort's darkness can't influence Riku's heart and Riku's darkness anymore.
Young Xehanort explained that to Riku when they talked in the throne room. He also explained that he had to give up on using Roxas as the 13th vessel because he developed a too independent sense of self and will.
Which is why he now resorts to try and use Sora.
In Re: Coded Data-Naminé warned that all the suffering and pain of the tormented ones (TAV, Roxas, Xion and Naminé herself) can overwhelm Sora's heart and cause it to fall to darkness.
It appears that Young Xehanort and Xemnas know this fact too and try to deliberately make that happen in Dream Drop Distance, causing Sora eventually to fall when it becomes too much for his heart to handle.
The reason Xehanort gave up on Riku is because he developed a strong resistance to darkness. Meaning that Xehanort's darkness can't influence Riku's heart and Riku's darkness anymore.
Young Xehanort explained that to Riku when they talked in the throne room. He also explained that he had to give up on using Roxas as the 13th vessel because he developed a too independent sense of self and will.
Which is why he now resorts to try and use Sora.
In Re: Coded Data-Naminé warned that all the suffering and pain of the tormented ones (TAV, Roxas, Xion and Naminé herself) can overwhelm Sora's heart and cause it to fall to darkness.
It appears that Young Xehanort and Xemnas know this fact too and try to deliberately make that happen in Dream Drop Distance, causing Sora eventually to fall when it becomes too much for his heart to handle.