A leftist monopoly on broadcasting occurs because the Left is able to intimidate advertisers who dare to have their ads on “objectionable” shows.
Because rightists are busy producing.
Leftists flock to non-productive areas that serve as propaganda outlets (i.e., propaganda is their only real product):
Media. Academia. Politics.
And because all the networks are located in major Cities and their hiring pool is made up of 90% liberal Democrats. Demands for conformity then weed out dissenters who might challenge the dominant orthodoxy.
The media is liberal because everyone who works there is liberal, right down to the janitors that clean their floors.