It's traditional:) The original machines with simpler hardware largely converged to BASIC, so it's a nice benchmark for "is this a Real Computer™"
Every machine ran BASIC. If your machine didn't, it was weird. For a while it seemed like BASIC was going to become the universal computer language.
IBM even produced a computer that had both AP/L and BASIC built-in.
Aha, step forward the Jupiter Ace...