Here's why:
http://sports.yahoo.com/news/pft-nfl-network-cancels-airing-200235656.html
PFT: NFL Network Cancels Re-Airing of Rams vs. Browns Game
Why? This was the reason according to Florio:
So why did NFL Network dump the Rams-Browns contest? Per a source with knowledge of the situation, the Browns disregarded the new ban on virtual signage during preseason contests. Which prompted NFLN to in turn disregard the Browns’ broadcast.
Virtual signage are advertisement images that do not physically exist at the stadium but are added to the broadcast film via computer. These often appear as logos projected onto the field. Per Florio, the Browns' version of the broadcast had some of these images and was the copy of the game that NFL Network was scheduled to air. The NFL has a rule against using virtual signage in preseason telecasts. Due to the violation of this rule, NFLN canceled their re-airing of the game and will now wait until they can obtain a copy of the St. Louis Rams' broadcast of the game.