OpenSSL's API has a lot of surface area. As someone who has used it on a project, once you start trying to do "advanced" things like optimize round trips in co

OpenSSL's API has a lot of surface area. As someone who has used it on a project, once you start trying to do "advanced" things like optimize round trips in conjunction with your application layer on top of it, you have to start poking around some obscure parts of that API, where the only documentation is "read the source code". Even some basic stuff like validating that the certificate corresponds to the domain you connected to (like, pretty important!) was historically not done by default and required interfacing with a bunch of low-level stuff. The things you used to have to do to use the Windows system certificate store were also pretty hideous. I believe these things are handled better in more recent versions, but of course much software was written in the past, including mine, and if you wanted it to work (and continue to work, with older versions), you had to do the hideous things. So a drop-in replacement is really a tall order, and would also require repeating many of OpenSSL's mistakes to achieve true compatibility. Ironically OpenSSL 1.2 itself broke a lot of these APIs in ways that affected my project (and were not, in my opinion, always strictly better). It's a mess.