Well, I think we all know I'm gonna say this, and I refuse to except that answer And here's why
1)got dammit the ellimist did not develop everyone and everything down to the last detail in the ani-verse., no dammit no no no *rambles*
2)Now, for a more rationale, and less hostile reason: If memory serves, the ellimist used it totally by accident the first time, and sign language was still common for andalites when he left there.
I think it was an innately andalite trait, he was just the first to tap into it.
As to how they got it: well evolution's a trip like that. I can't really imagine the physio or physics behind it
but yeah, they just got it and kept it. All I can think of