Want to feel old? Windows 7 came out 14 years ago, and 8.1 is already over 10 years old. Sure felt like it wasn't that long ago when these replaced XP and Vista, but here we are. Microsoft themselves have cut support for these systems a while ago, so it was only a question of time before Steam would too.
Valve cites multiple reasons for this decision. The most obvious one is that some of Steam's core features rely on Google Chrome, and that's out of commission on older versions of Windows.
The other reason is more pragmatic. Microsoft is not patching and updating those older Windows systems, which creates all sorts of risks and vulnerabilities. That's something that any company typically tries to avoid, but Valve in particular also seeks to utilize new features of W10 and W11 in the future.