No one has ever commented on the change specifically.   You could ask Gygax on enworld, I suppose.  

The current theory is that they were able to increase production values based on the enthusiastic sales for the hardcovers and monochrome modules.  Modules were printed on demand, until they sold out, then another order was made.  By 1980, sales figures were high enough that TSR was turning heads.  This culminated in the "color reprint" phase (D1-2, D3, S1, S2, etc.) of 1981-1982.

B2 itself was never released as a monochrome; it was one of the first full-color-cover modules.  But you can see an interesting pre-production mockup of it on the back of the 1979 Basic Set, or here at the Acaeum.