The reason: it's not ready.
“We are not satisfied yet with Dawnguard’s performance on the PS3,” said the company in a statement on its blog. “We would like for everyone to have a chance to play Dawnguard, but we aren’t going to release it for PS3 knowing that some people’s experience in Skyrim will be worse. “
We have come to expect, and to some extent forgive, some of the issues that plague the traditionally ambitious Bethesda Game Software releases. Skyrim had serious problems on PlayStation 3, however, which is why it technically wasn’t part of our Game of the Year award for Skyrim last year.
“We do everything we can to have our content available to all; from our free updates, to user mods, to paid DLC,” continued the statement. “We’re as disappointed as our fans when that isn’t the case, but we’ll continue to push for that reality.”