It was always intended to be on the shuttle. It's not as if they conceived, designed, and then created it between STS-1 and STS-2. It was baked into the software and into the rear flight deck controls. It was late.

They didn't call it "flying the arm" for nothing:

https://www.alamy.com/stock-image-sts077-307-017-19-29-may-1...

In any case, STS-1 was an insane test flight, and had it's own share of thermal tile problems.