Well, after a whole minute and half of thought I can't think of a logical conclusion, so I'll give you my conspiracy explanation:
Stainless strings have better shelf life. Phosphor bronze strings lose their shine rather quickly, maybe too quickly if a bass spends more than a couple weeks in transit and storage before it sells. For a standard acoustic guitar it's no big deal for a dealer to change out a $5 set of strings, but bass strings add up.
Hey, you read it on the internet... it must be true.
