Forum threads like this one have been sprouting up like weeds on Bioware's official Star Wars: The Old Republic forum, causing mods to quickly open new ones due to the overwhelming amount of responses. The basic idea of threads like these is simple -- EA has too many servers, and as a result, players aren't finding anyone to play the game with.
One metric in the thread claims that even though The Old Republic has about a tenth of WoW's subscribers, it has nearly half of WoW's total server count, causing a massive server to player disparity. It seems as if EA was outright expecting that The Old Republic was going to kill Blizzard's best straight out of the gate, but that overestimation has punished the players, forcing many Republic users to deal with ghost servers.
I can personally attest that the more I leveled up, the less people there would be in any given area; even on the Empire's side (which has more players on nearly every server). I have a feeling once EA releases official numbers for active subscribers (that bothered to re-sub after the first free month) in the coming months, it's not going to be pretty.