The problem with political parties that offer their followers an identity is that identities are inherently conservative.
If a certain attitude – say, animosity – is what defines your identity, you will be reluctant to change your attitude even when the situation around you calls for a different response.
Most of the time, people don’t want to stop being who they are. Yet most of the time, the world is changing. This constant of change is especially true of populous, multicultural liberal democracies as well as in international relations.
The change-averse nature of identity also means individuals will create feedback loops with the people around them so as to remain “in character”. If a person’s political identity is defined by, say, alienation, they will pursue discord so as to validate their worldview and thus preserve their identity.