As I understand it the distance of the centre of gravities from the fore and aft roll axis is one of the components of stability.
Normally (and please correct me if I am wrong) stability in a boat is calculated by the multiplying the weight of each component by the distance from the roll centre to get a moment.
Moments can be positive or negative.
As a boat starts to roll the water
displaced exerts a positive moment (tending to move the boat back towards the upright position). THe weight of the keel on a mono has little effect at this point.
In a catamaran
the initial positive righting moment is large as the distance of the displaced water
from the roll axis is large (compared to a mono).
However as a boat continues to tilt it reaches a point where the righting moment from displaced water starts to diminish.
For a catamaran
the negative moment of the mast
now tends towards being larger than the positive moment from the displaced water and the boat tends towards being negativly stable.
For a mono the moment due to the weighted keel should now be larger than the negative moment of the mast
If a yacht should become perfectly inverted then there is no moment to return it to the upright position. Narrow monos need only a small variation from this inverted position to become positivly stable again.
If we consider more and more beamy vessels the variation from the upright inverted position needed to acheve a positive righting moment becomes larger and larger to a point where it would normally not happen on many catamarans.
You can find a link to the stability diagram for a typical modern catamaran/trimaran (John Shutleworth)
and a mono (Dudley Dix - Didi 26)
Note that the catamaran/trimaran becomes negativly stable at 85 degrees while the mono becomes negativly stable at 120 degrees.