The reason for this is due to Steam's dependence on an embedded version of Google Chrome that no longer works with older versions of macOS.
Valve announced that it will stop supporting macOS Lion (10.7), Mountain Lion (10.8), Mavericks (10.9), and Yosemite (10.10) from January 1. The world's largest digital PC game store Steam will stop working with specific versions of macOS from January 1, 2019.