Developers want to maximize profit. Bay windows don't need to pay URA but buyer must pay floor area of all bay windows.
You will notice that if the developers (before 2009 planning) line all the perimeter of the apartment with bay windows. Multiply 1 ft width times 120 ft of perimeter of windows they can charge buyers 120 sqft x $1000 psf and not pay dev't charges to URA.
Glass windows above bay area hotter than brick walls, right?
In fact the more developer try to charge the buyers, the more glass windows they must installed. Particularly when these windows faces W/E direction, the rooms get very very hot with noon sun.
Go to URA site and read the report why developers have been abusing this rule resulting in very hot buildings. Hence bay windows will be optional in future.