Apple has a history of supporting only the three most recent macOS releases. When we release new products, we support only the operating systems that are supported by their manufacturers.
We also find that some of the most frustrating and impossible-to-fix problems - that also happen to cause us a lot of technical support effort - are on the oldest platforms. Our inability to fix problems on unsupported platforms, combined with the pain and cost of supporting them, leads us to not support the platform, either.