Because it means that the customer then gets better products in future at less cost.
Or the inherent cost of making Windows has gone up, and that removing expensive components has meant that Microsoft didn't have to pass that cost onto consumers.
Or maybe the reduction in price allowed Microsoft to subsidize Windows to $40 for six months for millions of customers.
Don't forget inflation too. If something new is the same price as the thing it replaces when it was sold 5 years ago then in real terms it's cheaper.