I'm far from being an academic, but if I were to make a less snarky interpretation, "not unreasonable" would be written if the reasonableness isn't binary, meaning that "not unreasonable" doesn't necessarily mean "reasonable", and that interchanging the two might lead to a less accurate, stronger statement. Alternatively, one would write "not unreasonable" as a way of contrast relative to something that was previously claimed to be "unreasonable".

I can think lots of other reasons, but feeling clever isn't one of them. I mean, if someone would like to feel clever, there's probably a more eloquent synonym for that.