The difference is that in Tunisia the Islamists have to form a coalition government with a social-liberal party.
A system in which the liberal party keeps the Islamists in check is preferable to a system in which the army keeps the Islamists in check.
Situation in Morocco is similar to Tunisia's btw, except there also the monarchy in addition to the secular parties with thom the Islamists have to govern now.
Might be true in Egypt, as well, as I don't think the Salafis and Muslim Brotherhood are fond of one another.
Also, given the fragmentation of the secular parties, I doubt Ennahda actually has to form a coalition with any of them. They could definitely run an ad hoc minority government. (Also, hesitant to call them all liberal, as the secular parties include things like the socialists and communists as well as the liberals, moderates and social democrats.)
Nobody in Tunisia wants a monolithic minority government which would be naturally unstable. It's exactly what a country exiting from dictatorship needs now.