Because they don't believe the increase in hardware sales will provide an increase in hardware revenue/ profit.
They are clearly ok with sales as they are.
And why don't they believe that?
More PS+ subscribers is already enough reason to drop that price.
I can understand the PS3 as it was never that popular and PS+ was still small there, but now most sales come from PSN, and having a PS4 give people access to a whole lot of games to buy there.
I can't see why not just do it.