A better way of looking at this is from the perspective of where they make money: targeting Windows makes sense because it's core to Microsoft's business, their reputation is heavily linked to it via all of those enterprise contracts, and their revenue scales directly with the number of people using Windows.

In contrast, Flash was an Adobe acquisition which they never had a clear vision for and the model of selling tools but giving away the runtime for free meant that your app being popular didn't make more money for them. Anything security-exposed like this needs substantial amounts of ongoing maintenance funding but from Adobe's perspective that was mostly overhead or something Google would subsidize for them.