Because making it on a modified J frame with a K cylinder would be smaller and lighter!

I grew up shooting Detective Specials and found they shot like a 4" S&W.
So I preferred Colt DS's over Chief's. I'm still better shooting a Det Spl than a J frame.

Yes, they were heavier, but the ballance was right and thusly they shot where
they pointed. If I could find one of the last DS's I'd jump on it.