1. ISPs have been given various money to roll out to those communities in many if not most cases. Currently, noncompliance is cheaper than doing what they said they'd do and were paid to do; and
2. The "rational" thing you point to allows racism by proxy. I mean, look up redlining. Systemic discrimination (of various kinds, not just race) gets more abstract but the end goals remain the same.
But really the FCC is trying to put a bandaid on a gaping wound: national ISPs shouldn't exist. The only good broadband is municipal broadband. Any "good" broadband we have now just isn't terrible yet and/or the provider has cherrypicked coverage in much the way you suggest, leaving many under-served communities.