When we're talking about ads and media, I highly recommend reading the relevant chapter in Manufacturing Consent (PDF version can easily be found for free online).

But really, intuition will get you the raw basics: using the ad revenue model gives the advertiser control over a media outlet. If media truly 'need gambling ads', then this implies they cannot afford to lose them. So, they therefore cannot offend the gambling industry or especially the companies advertising with them. And therefore, they are pressured into media bias, into failing to be critical of an obviously harmful, corrupt industry dealing in addiction manufacture AND laundering at the same time!