I write and have fun. I have been playing since 3002, I mean 2003. I started with DnD 3.5 while staffed at a Cub Scout Camp. I played in college as DnD 4 was releasing but missed it. I've been playing 40k games like Dark Heresy and Rogue Trader, and Iron Kingdoms. I've picked back up on dnd with 5e. Campaign is going good so far.
I'm the best. Impossible questions I have answered.
Why is this [blowgun] weapon listed as a martial weapon? ...My perspective on this has led me to conclude that it must have been a mistake...nly the designers would be able to shed some insight
This is really your problem. You've blocked out the correct answer and asked for another. It is in the martial weapons because only trained people would use it and it isn't common.
Shortbows are common. They are used by everyday people for hunting and widely available.
A trained rogue can stay concealed and use a blow dart to knock out guards. The same rogue would have to blow her stealth if they had to stand up with a bow or use a loud crossbow. If a dart misses, the targets are less likely to notice. DnD is a game of imagination and creativity and has decades of thought put into it. This isn't a mistake, this is a personal mental block that you have.
One more thing: don't spend too much time merely reading. The best part of this work is the play, so play and enjoy! Gary Gygax
"It's the first game designed knowing that it is D and D...we build a game that makes that answer true, like as true as possible, and making those sorts of tropes...like fireball is a third-level spell. Damage-wise, math-wise it's too powerful but we did that by design because oh fireball getting fireball is awesome!"Mike Mearls A 5e roundtable Blockquote
The simple answer is because it feels like D&D. It was in D&D before.