I'm not sure his audience is that big. Cable TV audiences are big and consistent, and they bring big advertising dollars. I'm not sure if 1.5M views is that much.
When a network backs you, the advertising, promotion, reach is substantial.
Just because someone is on YouTube and theoretically 'anyone' can see him is not actually material reach.
When you're making a BBC/Sky show, that opens up doors to significantly more places, whereas today he's just 'some youtuber' which is nice, but it's not big.
And of course follow on deals and residuals.
For example: he gets booked on the Talk Show circuit, BBC interview circuit, his profile is raised. He maybe can get access to CERN, or the Moscow Ballet or whatever, things like that.
The various auto enthusiast shows from the UK are massive. Those hosts make a lot of money.