Professional courtesy. So not to step on LLL's toes? If you want to be more cynical, it's not a good idea because they all be undercutting each others potential sales.
Many of the scores are premiere releases or available for the first time on CD - meaning nobody had those CDs already.
I had The Fly, and the boxset presentation of Nightmare on Elm St (although not this version, the OST). However, 10 other horror scores for $50 is a great deal. I snatched it up.