Sites targeting tech-savvy users are more likely to use HTTPS because they know their users will notice it, tech-savvy users are more likely to use ad blockers. More of a correlation than a causation. At least, I assume that's the theory. Seems like the effect would be small at this point though.

HTTPS requires 2 extra round trips, if your servers aren't close to the users, or the users are on a high latency connection anyway, https adds a real delay. HTTP/2 multiplexing and push /may/ help get some of that back, but I don't know if there's anything that does HTTP/2 push well in a non-custom server way yet?

If you configure your server to use TLS resumption and False Start there should only be one extra roundtrip, which can make a significant difference.

https://istlsfastyet.com

Apart for the other reasons provided by others, there is also the issue that sites using https are more likely to have savvier users and so may be less likely to purchase after clicking on a link. An advertiser is charged per click, but their return is the conversion rate on those clicks to sales.