I work for google, and i had a fair amount of work on moving Ads to HTTPS (moved mobile app ads to https)

The work actually started quite a while back, but the overall ads industry and internet as a whole moves really really slow. Add the mobile ecosystem to the equation, and there is a bunch of issues.

The whole work is a combination of a bunch of things (in no chronological order): 1. Google pushed search ranking changes. 2. Google moved all of ads to HTTPS, and this took some time to make it happen. 3. Apple created ATS to make people think about it. 4. Apple wanted to enforce ATS for non-web content, had to back out. 5. Let's encrypt made access to certs free. 6. Big vendors joined.

Unfortunately, the world is slow when it comes to changes like these, but i am quite happy with the outcome so far.

edit: added context.

How early? Put a year on it. What should they have done, and when?

If Google hasn't threatened people that ranking will sink if they didn't migrate to HTTPS, Let's Encrypt wouldn't have been so popular despite the fact it's free due to the pseudointellecutal SEO "muh performance hit, muh redirect rules" meme