Let's Encrypt is still far from being convenient and usable for small companies or individuals that don't want to manage infrastructures and administration like writing a cron job that refreshes your certificate.
The lack of raised hands, should answer for you why LetsEncrypt is not ready for mainstream use.
Until Cert renewal is baked into all the hosting apps/platforms - HTTPs will never be ubiquitous.
Not that they host it with their own computers, but they rent some space online and host it there.
EDIT: Replied to the wrong parent, please ignore in this context.
EDIT: You can't delete comments with replies anymore. It's very annoying.