The Bengali nation struggled for democracy, secular values and national rights for years. Military rulers of Islamic Republic of Pakistan tried to deny democracy and national aspiration of Bengalis and even carried out systematic genocide in the name of religion. As a result, struggle of Bengali people from language movement of 1952 to armed resistance in 1971, lead to emergence of Bangladesh as secular democratic nation state.
So, following fundamental principles of Bangladesh constitution of 1972 evolved through experiences of this popular struggle.
(Socialism was other fundamental principle. However this principle was generally considered for social justice particularly for the disadvantaged.)
Bangladesh people are still continuing the struggle to retain these principles against many odds.
Source: Liberation War Museum