Why is there no option to close apps in the multitasking menu? This is another tweak that many users have been requesting. Whether you tap a cross or you can swipe them away, the ability to close running apps from this menu would be good. No one wants to have to go into the app in order to close it.
Hopmedic answered your question
here. In a nutshell... because there is no technical necessity for it. Furthermore, and on a somewhat abstracter note:
At the core of the WP experience lies the idea, that users shouldn't be required to micromanage any aspect of their phone.
That rule is particularly important for scenarios that are purely technical in nature, like manually managing app lifecycles. If you forget everything you know about computers (and Android) and think only about the things you use a smartphone for and how it should function, having to manually close apps starts to sound rather absurd, does it not?
Anyway, the "only" reason people desire a close-feature (beyond it being familiar), is because they're looking for a way to better deal with poorly designed apps (i.e. having to spam the back button to close them) or some OS level bug. While the demand for a close-button is understandable, adding an OS level feature specifically to ease "defect management", is a terrible idea. That is similar to being sick, but dealing only with the symptoms instead of curing the illness.
For all the reasons I've seen mentioned so far (including those mentioned in this thread), there are better and director solutions, which solve those issues at their source, instead of doing so indirectly via an OS supported close-feature.
Don't get me wrong. I'm not criticizing anyone for wanting a close-feature. I understand the need. The close-feature just isn't the right solution to those problems...