The Problem
Ok, so to begin with, Cersei still has some outstanding charges against her, which simply won't go away:
- Adultery
- Fornication
- Incest
She could order everyone to be killed, but the problem is that these are charges laid upon her by The Faith. In their custom, The Faith is still an extremely important part of their culture.
You saw the way that the "smallfolk" (commoners) behaved towards her during her Walk of Shame. Now imagine if she was to order the death of like hundreds even thousands of people. There would be outrage and "the many" (smallfolk) would realise that they can stand-up against "the few" (the Lords).
The easiest road
Now, at this point, all she has to do, is name a Champion, who would defend her. And given that her champion seems to be Ser Robert Strong, who is a humongous Knight, theorised to be
an undead Gregor The Mountain Clegane[1]
who would most likely defeat any known or probably even any possibly champions for The Faith, and just be done with all of her problems in one go.
It's a huge gamble, but Cersei is insanely over-confident and well, just plain, insane. (but she's not so insane as to kill everybody). And given that the outcome of a Trial By Combat is binding, once her champion wins, she is no longer to be pestered by these accusations. See What are the rules of Trial by Combat in Game of Thrones?.
Reasons for The Hype
[This part of my answer will contain spoilers for A Dance With Dragons and post Season Five of A Game of Thrones]
There is a theory called Cleganebowl which hypothesises that The Grave Digger found at The Quiet Isle by Brienne and Pod is none other than our very own Sandor The Hound Clegane[2] and since he's repentant towards The Faith, it is highly believed that he will be the one to champion The Faith against Ser Robert Strong (read #Cleganebowl #GetHype)