Because emotional expression is OK in humans? That complete suppression of emotions can be problematic for humans, and her going through some seriously trauma impacted her ability to self-regulate. Basically, she had trauma, wasn't allowed to process it, and then was thrust back in to a situation were she was exposed to a trigger causing further dysregulation.For someone raised by Vulcans she is still the most emotional character in all of Star Trek.
tl:dr-I would not expect Burnham to be able to maintain her regulation despite being raised by Vulcans.