Why would the Speedbrake be required for such a long time on approach?
There are two reasons I can think of off the bat: 


Its possible the controller asked them to hold a slow speed due to increased traffic ahead. The brakes may have been deployed to match the speed requested. I have heard this called on occasion over the radio in the terminal area I fly.  
The controller cleared them for a steeper decent than usual this would cause them to potentially deploy the speedbrakes to make the descent rate. 


This article discusses it in reference to small planes but idea is the same. And another article by a CFI here.