From what I have been told by old-time Chicago sports fans:
Comiskey was more popular fan wise than Wrigley until the early 80s.
At that time, Cablevision and Harry Caray contributed to more casual fans becoming Cubs fans than Sox fans.
It wasn't until the early 80s when stories of dangerous happenings around Comiskey, (what % actually true, who's to know without a library trip) contributed to suburban, Iowa, and Indiana tourists taking their tourism dollars north.
The casual fan saw scenes of Disco Demolition and housing projects and deemed the area around Comiskey to be more dangerous than it probably was.
Also, the general mediocrity and decline of the team, neighborhood and ballpark caused sharp declines in fans that never have fully recovered until the "antiseptic" park of USCF was remodeled and the casual fans had a solid reason to come back(ie. winning).
In short, "Ferris Bueller", Jim Belushi, Harry Caray, WGN on cable nationwide, a decrepit ballpark and declining neighborhood on the Southside, combined with 1984-1990 being poorer years for the Sox, (conversely, pretty positive years for the Cubs) has led to most casual fans between the ages of 15 and 40 being drawn stronger toward the more "appealing" ballpark experience on the Northside.
Because more casual fans find the Cubs more appealing, TV and newspapers show and print what they think that that casual fan wants to read.
No conspiracy, just supply and demand.
If the Trib ever sells the Cubs and the Sox continue to have years of good teams, the Cubs will be the second team again.