There are a lot of factors that went into it including:
- Browser and server technology for security with virtual hosts. You used to need a dedicated IP address per secure site, but that is not the case anymore using SNI.
- Lower cost security certificates. There are even free ones available for some single domain cases. Ten years ago I was looking at $300/year for a wildcard domain, but now I can get a certificate that includes wildcards for several domains for $70/year.
- Overhead of HTTPS dropped significantly. It used to require extra server resources, but now the overhead is negligibleoverhead is negligible. It is even often built into load balancers that can talk HTTP to backend servers.
- Ad networks such as AdSense started supporting HTTPS. A few years ago, it was not possible to monetize an HTTPS website with most ad networks.
- Google announcing HTTPS as a ranking factor.
- Big companies like Facebook and Google that moved to HTTPS for everything normalized the practice.
- Browsers are starting to warn about HTTP being insecure.
For big companies like Google that could always afford to move to HTTPS I think there were a couple of things that pushed them to implement it:
- Leaking of competitive intelligence data over HTTP. I believe that Google moved to HTTPS in large part because so many ISPs and competitors were looking at what users were searching over HTTP. Keeping search engine queries under wraps was a big motivation for Google.
- Rise of malware targeting sites like Google and Facebook. HTTPS makes it harder for malware to intercept browser requests and inject ads or redirect users.
There are also some reasons that you are seeing HTTPS more often in cases where both work:
- Google is preferring to index the HTTPS version when the HTTP version also works
- Many folks have the HTTPS Everywhere browser plugin that automatically has them use HTTPS sites when available. That means that those users also create new links to HTTPS sites
- More sites are redirecting to HTTPS because of security and privacy concerns.