Tony..I'm no expert but my understanding is that it has something to do with the dispersion pattern..
Speakers that have a vertical array of drivers, radiate more sound to the sides than up or down, when mounted in the vertical position..
This of course helps to give a good sound stage across a screen, from your L&R speakers..
Likewise for surround speakers..They need to be placed in a manner that will give the best flow of sound from front to back..
If they are designed to be mounted vertically (as most surrounds are), mounting them horizontally will diminish that flow of sound..
Others more experienced than myself might have other ideas though..
Speakers that have a vertical array of drivers, radiate more sound to the sides than up or down, when mounted in the vertical position..
This of course helps to give a good sound stage across a screen, from your L&R speakers..
Likewise for surround speakers..They need to be placed in a manner that will give the best flow of sound from front to back..
If they are designed to be mounted vertically (as most surrounds are), mounting them horizontally will diminish that flow of sound..
Others more experienced than myself might have other ideas though..