Because Bill Gates is on the top of the mountain and every hacker in the world wants to kick him off. If everyone was focusing on Apple, problems would be exposed there, too.
There's no such thing as a hack-proof system...there's always a better hacker. The only way to make a system completely secure is to disconnect it from any form of network communication. And even then, you need to worry about securing the room.
Sorry but Microsoft has admitted that Windows was not designed with security in mind. Its not market share that is Microsoft's problem with Windows, its the design. VP Valentine said as much in a speech. The market share excuse is a worn out Microsoft marketing ploy.