That all started with Roy Masters manipulating the media with that Silvertails v Fibro bullshit which the public lapped up.
Yeh he had a big hand in it but there really is a host of factors I think namely;
* Northern Beaches perceived as insular, snobbish and rich area (I grew up there and this isn't really the full reality apart from maybe the insular bit, but anyway) .
* Very successful in 70's, also up near the top throughout the 80's and 90's.
* Raided clubs (namely Souths and Wests) for top players in 70's-80's
* Richest club in 70's-80's
* Manly heart and soul Arko head of ARL 80-90's, perception of NSWRL/ARL bias to Manly
* 1978 finals campaign, Greg Hartley etc.
* Bob Fulton, a winner but pretty abrasive, rubbed a lot of people up the wrong way.
* We had a host of real arrogant dudes playing for us when we were dominant in the mid 90's...Terry Hill, Matt Ridge, John Hopoate etc.
* Roy Masters Silvertails thing.
I dunno, what am I missing?