I could name at least 3 shows that had "Petticoat" in its title...

Petticoat Junction
Operation Petticoat
Pistols & Petticoats

What's the deal with that?

Why was that a popular name of tv shows in the 60's/70's?

There's a wiki article (http://en.wikipedia.org/wiki/Petticoat) that explains what it is. You'll notice that it seems to go to great pains to not mention Petticoat Junction in the Popular Culture section at the end.

Petticoats had begun to be considered an old-fashioned accessory years before PJ came along, pretty much disappearing from common dress right around the same time the series began. Therefor a petticoat being a nostalgic item was certainly not out of step with the character of the series as envisioned by the pilot.

If you think the show ended up with an unlikely title, well, you're right I suppose but when you look at the other working titles--Whistle Stop, Ozark Widow, Dern Tootin' (fer crying out loud!)--Petticoat Junction starts to seem awfully clever by comparison.

A petticoat naturally invokes thoughts of a feminine element as well. All three shows you mention were either strongly nostalgic or period pieces and all had a mostly female cast. So there y'are.