Maybe the price? I reckon they'll add more customisation options later on.
It could be. Could also be that Dyneema gridstop has better properties, Bonfus states they use it for 'flexibiliy and durability' SWD for 'tear resistance' in those same areas.
Could also be that hip belts, shoulder straps, and side pockets are standard parts and they have hundreds in stock they don't want to waste.
Maybe it's trickier to work with and those areas might be more difficult to make.
Or some other reason. I don't know, but I'd like to find out before I get a pack made of it. I suppose it will take a while before it's known.