The original ones were only spec'd to 400 MHz, top of the line now is 550 MHz
The device may well run at significantly higher than 280 MHz, depends on what process it's built on, perhaps they have a slow one to address some specific customer (Nintendo, White Goods?) or niche market where the price/speed tier fit? Or it competes against as specific Microchip model.
The ability to fab at an old facility might have some benefits at this point, especially if those offering tighter geometries are "busy"
Either way it's a business decision, and really doesn't need to be justified. If you need a faster part, pick one.
