Transact SQL :: ALTER Vs UPDATE When Creating A New Column With Default Value

I have a table with ~30M records. I'm trying to add a column to the existing table with default value and have noticed following ... When using alter with default value- (Executes more than 45 min and killed forcefully)

ex:  
ALTER TABLE dbo.Table_X Add is_Active BIT CONSTRAINT DF_Table_X_is_Active DEFAULT 'FALSE' NOT NULL
GO

When using update command after adding column with alter (without default value) it completes is 5 min.

ex:  
ALTER TABLE dbo.Table_X Add is_Active BIT NULL
GO
UPDATE Table_X SET is_Active = 0 WHERE is_Active IS NULL
GO

Why there is so much of difference in execution times ? I was just trying to understand internal behavior of the SQL in these two scenarios.