I've looked at an old map showing night DA patterns for every station on 970 ... WTBF's main nulls seem to have been occasioned by Louisville KY (north), Tampa FL (SE) and Alexandria LA (W-SW).
Louisville was the strongest factor, at least where I lived (edge of one of the nulls). Sometimes they walked on WTBF considerably ... there was one night I worked, and before I left the house I started a VCR feeding from a GE Superadio III, in the one spot I was able to receive it with a minimal amount of hash. It was a special night, I recall, and wanted to do a full uninterrupted aircheck.
Anyway, I was playing it back and during the last hour - over a song with a soft passage, a commercial for WLKY "News Channel 32" (ABC affil, Louisville) was clearly audible! Crazy.
Another couple of factors endangering directionals: real estate values, city growth (i.e. sprawl now filling a null area, and much of these residents are your P1s), and - especially - the engineers who have the experience with setting up directional patterns are retired and dying off. I can't imagine a whole lot of the new breed of radio engineer possessing this kind of knowledge ... and those who do are at the larger AMs.