My question is as title stated.
Why mac osx user directory located at
/Users, but not
/home is the default directory of user's home of Unix and Linux.
While we can only guess on Apple's motivations for certain decisions, the most obvious explanation would be that a "Users" folder has existed since Mac OS 9, before it was a Unix system, and Apple chose to stick with something familiar to their users. The same goes for other already familiar folders like "Applications".
This effectively translates in two different parts of the filesystem, where the not-so-user-friendly Unix hierarchy is hidden from the GUI, and a more friendly folder hierarchy is added on top of that and is exposed in the GUI.
There also is a clear difference in the style of directory names between the visible and invisible part. The invisible part uses all lowercase words as per convention in *nix filesystems, while the exposed part of the filesystem will uppercase the first letter of each word in a folder name.
/Users originated in NeXTSTEP/OpenStep, the ancestor of Mac OS X. /home is really just a Unix tradition and in no way necessary. You don't even need to have all home directories in the same containing directory. If you used Unix when HDDs were much smaller you might have seen something like /u0/user1, /u0/user2, /u1/user3….