That and the fact that wine only runs on *nix, so it'd require another compatibility layer to provide POSIX and BSD functionality under classic MacOS. OSX didn't even exist at the time IE for Mac was written. They'd also have to come up with a visual theme layer so they could at least pretend it's a Mac program, long before they introduced one in Windows. All of that still wouldn't get them menu bar support, though. In hindsight, I didn't think it through at all when I mentioned wine.
They do have MS-Office ports for Mac, so that product was probably written with much better separation of concerns.