Because they require constant reference to a state of exception, measures of security work towards a growing depoliticization of society. In the long run they are irreconcilable with democracy.
(Feature, not bug.)
Also this (previously):
In short, discipline wants to produce order, security wants to regulate disorder. Since measures of security can only function within a context of freedom of traffic, trade, and individual initiative, Foucault can show that the development of security accompanies the ideas of liberalism.
In combination, these two sentences provide almost everything political philosophy needs.