Because most slides are one directional. In other words, the cables slide on the plastic/teflon. On the ones that hold the strings on the inside of the guide rod if you were to just simply "flip over" the slide as you suggested you will see that the cables would be against the actual guide bar and not "riding" on the slide. The slide would just sitting there really not doing much.

The slides that are made to go on the outside of the guide rod have material between the rod and the cables for the cables to "slide" on.

Take a close look at some pictures of both and you will see what I'm talking about. Or better yet flip your inboard slide over and you will see first hand it won't work.

I think there are some companies that make aftermarket slides that you can use on either the inside or outside, but I think all factory slides are either inside or outside only slides.