Unless one has a true stereo setup there are lots of reasons. A lot of "stereo" setups are more accurately categorized as dual-mono setups. In such a setup the stereo image
is present to a few people in the middle. They basically resemble an isosceles triangle with their head and the speakers. To everyone else the stereo image
is not present, and so they are missing out on the sound that would come from the speaker
they are not hearing. With a true stereo setup there is even coverage from the left and the right everywhere in a venue
, and then there isn't a good reason not to, but with a dual-mono there is reason not to.
Also, in some cases one might select mono
over stereo so that they can bridge their amp/amps and get more power
out of it/them.