Because a product generally tries to solve one problem well and let other products solve other problems (which possibly don't even exist yet). The same reason your image editor doesn't help you process text, remaster audio samples, browse the internet or play games all at the same time.
Single responsibility principle?
Operating systems by their nature are not single responsibility software. I agree that the metro interface should have been toggleable but it really makes no difference in daily work on the computer. On my MBP without touch, I don't use metro. I use the desktop exclusively. On my surface, I only ever use the remote debugger on the desktop, everything else is metro. It does handle both sides well if you get over your bias and actually try it.
The operating system itself isn't. But the operating system's interface is. You just mentioned you do not use metro on your MBP (which does not have a touch screen). So how does "metro" handle both sides well, since you are not using it for your non-touch computer? Regardless, why would you put an interface clearly designed for touch screens on hardware which does not have touch screens? It does not make any sense. It's like saying you're going to be browsing the internet using a wiimote. It is incoherent.
Besides, having two radically different desktop interfaces in a single operating system is just more software bloat, a better alternative would be to purchase the core OS separately, and then select the interface you want based on your needs. Have a touch-screen? Grab the "metro user interface". You use a desktop? Get the traditional interface. It makes so much more sense than just lumping everything together and have the user jump through hoops to disable (and not even completely remove) metro. Yes, it takes more effort to maintain, well, there's no free lunch, you can't maintain two radically different products by simply assuming they are, in fact, equivalent. Sorry.