Because the Spade is awesome?
The only thing that beats a Spade at close range is a Shotgun (and arguably an SMG, but you actually have to be aiming properly to hit with it at close range). It's a one-hit kill at point-blank, and it comes out pretty fast (obviously not faster than a bullet, but it can be spammed better than a Rifle, which is the worst weapon IMO against a Spade because it takes 3 or 4 slow hits to kill a person without a headshot).
It's also a very quiet killer, especially in comparison to Grenades and all guns, which make massive amounts of noise and give away your position. The sound of a Spade kill fades away around 10 blocks away, whereas gunshots and grenades can be heard from something like 50-60 blocks (across the Bridge Wars Map bridge at the very least). It's really good for tunnels and such because people won't know that there's someone hiding in there unless someone uses the Team Chat (which I hardly see anymore, for some reason).
Tbh, I think I started doing the spade thing after I got a bunch of kills with it in a single game, and then I started shouting "SPAAAAAAAAAAAADEEEEEEE", as a reference to the Space Core from Portal 2. Obviously this would take too long to type, so I shortened it to "SPADE".
EDIT:

Was running around, one person spotted me so I was forced to kill them with the rifle, but someone else was digging a trench so I ran in and Spaded them.
Also shown is the vulnerability I show as soon as I get a kill with the Spade. The mass of blue is the Blue corpse I Spaded, but I fell on top of it face forward because someone got a headshot on me while I was typing. :P