Why can't mortise locks be installed upside down?

I purchased a mortise lock for a gate today and the person at the shop told me to make sure I install it right side up — lock on top and handle on bottom — and that a lot of customers install them upside down then complain that they don’t work correctly.

I believe them because this shop existed for the sole purpose of making custom doors and selling parts, but I can’t really figure out why and didn’t think to ask when I was there (and also I don’t have any pressing need to install one with the lock on the bottom).

My question is: Why don’t mortise locks work correctly if installed upside down (handle on top, lock on bottom)?