The reason is back when the A2000 first came out, Commodore had a trade up program. You would turn in your A1000 keyboard (thus making your A1000 inop) and you would get like ~$1000 off the price of a new A2000 system! What a great deal!!!! 
Exactly, and no one ever found out what happened to those keyboards...
Although technically the A1000's keyboard interface is exactly the same as those used on the A2000/3000/4000 so it is only a matter of obtaining two connectors and wiring them up to make an adapter... I tried it and it works perfectly.
