There are a lot of factors to consider, but here's a few.
Advertisers prefer to advertise with the top tier of channels and do different forms of advertising with these creators.
There are more mobile users than ever, and this figure keeps increasing. Mobile advertising is not as popular as desktop advertising and has lower click through rates.
More and more people use blocking extensions for their desktops, you don't make money if the ad is blocked.
On top of this, there are more and more people opting into monetization after the partnership program was opened up to almost everyone. There's a lot of videos to display ads on.
Thankfully though, the number of YouTube users gets bigger every year, so this kind of balances everything out.