But everything is a tradeoff, and there are actual downsides here. Nowadays, Chrome and Firefox try pretty hard to make you think all non-HTTPS sites are bad for you. What about old web servers that people never bothered to update? They effectively become "broken" without actually breaking. There's tons of great information on sites without updated certs that becomes that much harder to access because of this.
There's also the concern about your actual codebase. Hypothetically, if you're building an HTTP client from scratch, you can write, know and understand every line of code in your project yourself. If you want HTTPS support, you're basically forced to link with OpenSSL, and this is almost guaranteed to take the total percentage of "your code" in your project from 100% to less than 10%. (Probably more like 1%.) You no longer actually know what's going on in your program anymore. Maybe this isn't a concern for most people, but it does matter to me -- and I suspect there are a lot of people who would care a bit more if they thought about it for a few seconds.
So yeah, HTTPS is great. But everything is more complicated than it seems.