I think there's a lot of historical accident involved. Personally, my experience of the two terms was that I heard about "services" aka "SOA" first, and it meant a top-down global architecture where the pieces were composed in elaborate, cumbersome ways: service buses, orchestrators, complicated XML-based technologies, service discovery, remote transactions, all that fancy stuff that many people tried and few people ever got working correctly. People were talking about integrating a new service to your architecture by dragging and dropping widgets in an orchestration dashboard generated from XML descriptors published by the services, and it was the kind of thing that blew your mind for a few seconds until you realized it was obviously never going to work.
Then I heard about "microservices," and the people using that term were talking about JSON and HTTP and DNS, stuff you could actually imagine working, and then you tried it and you actually could get it working with almost no effort, which was mind-blowing in a different and better way.
That difference was a historical accident based on where those terms were in the hype cycle. Now a lot of definitions of "microservices" describe architectures that are almost as elaborate as the old SOA ideas I was exposed to, and people have the same reaction I did to "services" back then. And now it's the supposed microservices experts who will tell you that if you did anything simple enough that you actually got it working in less than six months with less than fifty people, then you did it wrong and doomed your company. The hype cycle has made a complete turn.
