Because a file manager more or less would be a file explorer. Sooner or later someone would find a way to exploit it and get access to areas of the OS they don't belong. It could also decrease security, allowing potentially malicious apps to use this to access the system and do damage.
Not to mention, a Windows 8 style file chooser could solve just as much problems. Not all apps that support specific file types are probably listed as compatible in the store, so if you had a file manager anyways, you wouldn't be able to open the files in those unspecified apps, despite the fact they support them. However, if Microsoft implemented a (limited) file chooser, im sure plenty of devs would hop on board (im sure it wouldn't be difficult to implement), and it would allow applications, even those that are not default or those that are not listed in the store as apps that can open a specific file type, to open files they support. It also would only have limited access to areas (you wouldn't be able to explore everything. An app would only display files, from app directories, that are the specific file types the devs specify, and one would not be able to change the directory (since the file list would be pulling from all available directories), in theory.)
at least, that's the way I see it. Not to mention, there is alternatives to a file manager, such as the file chooser. A file manager is for opening and managing files, however, one doesn't need the multiple folders and such on a smartphone.
[SUP]Hope I explained what I meant clearly. I have a habit of confusing people[/SUP]