Probably because it looks like the infamous Graystone Martial Staff.
Imagine a lightbulb on a stick. Surrond said lightbulb with some metal which is pointy and appears to be out of tin. Light said lightbulb (now, the light hides some of the general uglyness of the weapon), and have a matching on the other side of the weapon and you have it.
The T5 bladestaff dosen't look that pretty either to be fair, but it lacks a lightbulb. However, itemisation developer dosen't appear to see any wrong with a bladestaff weapon using martial staff graphic so it's unlikely to change!
Irony of it all is that the handle looks quite decent, and if used alone, that would make a nice martial staff. Oh well, suppose higher tiers needs lightbulbs!

Here's the Bladestaff (not a great shot, but you get the idea).