Why is it so difficult to use a "true mirror" as a mirror

I am struggling to recall the scientific term for what you are describing, but it is a simple cognitive phenomenon that guides and informs the design of physical, real-world interfaces.

Let's look at a mouse on a computer. The motion of the mouse in the real world corresponds to the motion of the cursor on the screen. As an experiment, try to use the mouse sideways. (not upside-down). Notice how clumsy it is? It is because the motion does not at all correspond to what is happening in the real world. You are engaging another layer of your brain, a layer that is 100 times slower than the automatic layer engaged in movement of the mouse along its normal axis. This is irrespective of practice - it would still be more difficult if you had used the mouse in this manner for many years (unless you had a brain disorder).

The same problems confront folks who design computer interfaces for aircraft, spacecraft, and other complex physical systems. We have to design to decrease the cognitive load required to operate the device, and that involves, among other things, designing so that motion corresponds to the natural direction it does in your head. A mirror already complies with this principle (at least in the side-to-side plane), but a "true mirror" does not. That is the reason for the difficulty.

This answer supports the comment by Krysta that we are simply used to the mirrors we have and could just as easily learn to use a "true mirror".

In 1950, Theodor Erismann and Ivo Kohler performed a famous self-experiment in Innsbruck, Austria. Kohler wore a pair of glasses that turned his view of the world upside down continually for 124 days (sic). After about 8 days his perception had adapted and he was able to ride a bike or paint just like before.

The image below shows Kohler wearing the glasses:

Here is the original documentary movie created by the two researchers (in German) on the website of the Center for the History of Psychology at the University of Würzburg, Germany: http://www.awz.uni-wuerzburg.de/archiv/film_foto_tonarchiv/filmdokumente/th_erismann_ikohler/die_umkehrbrille_und_das_aufrechte_sehen/

Here is an article from the German Wikipedia describing the glasses and showing an image of a current (self-built) model: http://de.wikipedia.org/wiki/Umkehrbrille I don't know what this "Umkehrbrille" ("turn around glasses") are called in English.