Why did Cardinal block Yui from relating with players? I just finished watching SAO, and I'm not sure if this is ever explained:

Yui is a program that supports the psychological health of the players. She states that, for some reason, Cardinal did not permit her to relate with the players, and was limited to observing them.

Why did Cardinal block Yui? Is this explained in the novel perhaps?

If Kayaba created SAO, I'd guess that he also made Cardinal and Yui. I doubt that he'd want to block his own creation from doing its job, so I guess that the fault is entirely Cardinal's.
I just searched for the part in the novels - It's the third part of the second novel. But I'm sorry to disappoint you, there's not much explained:
(Source: Baka-Tsuki)

&quot;Although I am unaware of the complete details of what exactly happened as well, Cardinal handed down an unplanned order to me. A complete prohibition from interfering with all players[...]&quot;
Asuna reacted reflexively; she guessed that &quot;unplanned order&quot; was due to the manipulation done by the only GM of SAO, Kayaba Akihiko.

I guess it was developed by many people who planned to add this feature but Kayaba just decided to disable her without letting the other developers know.