Because she is the bestest warrior ever who easily beats the crap out of people when she is sick and when they are much bigger than her. She also can't be killed easily surviving situations she shouldn't via plot armor. The other characters are constantly talking about her even when she is off-screen. She is never truly guilty of anything because the plot won't allow it, see her recent evil clone drama. Lyta seems like a marry sue to me, but whatever you think.
Edit - If I had written the evil clone stuff I would have had it been Lyta then when she returned she'd have to once more earn everyone's trust (which is called character development based upon character flaws). A character who learns and grows is a good character whereas one who never struggles thus never learns is a mary sue.