[allovernow]

I suspect it doesn't really make a difference, there may have been one mediocre study which showed a couple percent improvement, or more than likely one opinion simply gained more traction than the other.

Both orders have downsides - if you floss first, you risk forcing detritus back between your teeth while brushing.

If you brush first, you risk leaving some of the gunk from between your teeth on the surfaces - although perhaps now it will be bound loosely enough to be washed off with water.

Either way you're probably getting rid of most of the organic material anyway.