Cortana, can no longer go rampant.
The stages the AI pass through are known as Melancholia, Anger, and Jealousy, also known as the Sadness, Rage, and Greed stages. A theoretical fourth stage, metastability, is also believed to exist.
In halo 3 on the second last level, she was in the 3rd stage of rampancy - Jealosy
When Master cheif rescues cortana at the end of Halo cortana enters the fourth stage:
Metastability
Metastability is a theoretical fourth stage of rampancy, where an AI can finally be said to be a true "person". While a metastable AI can be considered to be the holy grail of cybernetics research, there is no evidence to suggest than any rampant AI has ever achieved the metastable state, or that it is even possible to do so (with the exceptions of Cortana and Mendicant Bias).
So you see, Cortana can't go rampant because she already did, and reached the fourth and final stage