If the bracket only supports 280mm rads, then it should be pretty easy to buy/print/cut a 280 to 240 adapter so you can use 240mm rads.I'm assuming it's because 280 is fully supported. I'm sure someone will find a way for 240s to work.
If 280mm rads are the only ones supported, then I would think the reasoning is because they outperform 240 rads, so they're what most people would go with, so there's no reason to support something not a lot of people will use, especially if something is better.