The bike lane on Broadway is a test for operation and configuration. 
It only goes five blocks. Of course not a lot of people use it yet, it doesn't go anywhere yet. 
When it is extended to the Cherry Creek trail, and hopefully all the way downtown it will become useful for getting somewhere and people will use it. 

Plus, Broadway had far more capacity for cars than was being used.The amount of car traffic on Broadway fit easily in three lanes. All that extra space does is encourage faster driving, making the street more dangerous.