Why did Cardinal block Yui from relating with players?
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.