Why do software companies continue to support Windows 7 but not Snow Leopard? I do not understand.
Because windows is supported by Microsoft,and snow leopard is supported by Apple. It's two different companies,and many windows 7 computers cannot use win 8 or higher. Most snow leopard macs,that originally had snow leopard installed,works completely normal with sierra too. Windows 7 was in the beginning designed for small compact 10 inch laptops,and as long as there still is lots of those in use out there,they will support windows 7. Apple never built that kind of cheap,simple laptops,they made the iPad instead.
Sorry about my little mistake,might be language related.Minor correction:
Windows 7 is supported by Microsoft.
Apple has abandoned Snow Leopard.
Otherwise, your general point is correct. It is all based on how many active users there are for a given operating system.
There are far more Windows 7 machines still in use than there are Snow Leopard machines.
Even Apple will support Windows versions longer than they support their own MacOS versions. It's all about installed / active user base.