For things that touch the network and maybe particularly use TLS, sometimes people are suddenly in a hurry not to be using an old version anymore. The people with the classic compilers would probably have to be prepared to backport security fixes on short notice, I figure the curl project might not want to put them into that awkward position.
curl doesn't necessarily require OpenSSL for its TLS functions, in fact it's designed to be TLS-agnostic (to the point that it uses macOS Secure Transport API and Windows Schannel by default). You could either use wolfSSL or use a dedicated TLS code connected to the embedded cryptographic co-processor.
