Why is a third party likely to be an oppositional party?

Explanation:

Even in places where a third party is well established with representatives being elected, they normally do not have universal appeal.

Study Canada's system of electing representatives where there are 2 main parties and 3 others with varying degrees of representation.  The three just don't have enough universal appeal.

an informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.