Why does every state except Nebraska have a two-house legislature?

A practical reason for a bicameral system is to prevent the legislative branch from having too much power. It is supposed to provide for checks and balances and prevent potential abuses of power.

Until 1935, Nebraska had a bicameral legislature like many other states in the nation. It was argued that that a unicameral legislature would use a more fair and efficient system of lawmaking, would be less expensive and would do a better job at representing the citizens of Nebraska than the larger two-house body.

The answer would be: unhappiness