Because I've been in that position. I used to work the phones and promotions at the old Cool 94.3. I heard about the sale to Liberman through rumors going around in the station, not the suits.
A station sale tends to generate advance knowledge due to due diligence visits by potential buyers, etc. But in-house, same owner format changes are seldom known about in advance.
Sales, by virtue of the fact that they require FCC approval, of necessity are announced well in advance of the closing.
And small stations that are locally owend tend to be a lot more transparent, as the every action of the owner of the station.
And, if I recall, the owner was one Art Astor...