why i cant get my paid Left 4 Dead 2 back from steam support?

Of course they can't activate achievements. Otherwise they would have to deal with thousands of people complaining about "I earned this achievement, but it didn't activate, do it for me" and the system would both be flooded and fall apart if they actually listened to every sob story. Check the Steam store page to see if it says you own the game. If so, deactivate any DLC under the properties, since it is probably why the problem came up. If not, email Steam with a receipt or a reference to the order (which should be somewhere in your account history) and find L4D2 and contact them saying it isn't appearing in your account properly. Chances are that they either A. have to deal with dozens of these kinds of emails from people trying to scam the full version of the game from Valve by saying that it overwrote their purchased copy or B. misunderstood the question of wanting access to your paid copy as one of wanting achievement unlocks, and are saying no to you because of the achievements issue and not the product issue.

Everytime I contacted Steam support the first 2 responses were standard replies which completely missed the point. Make another ticket, maybe without mentioning the achievement issue. But as I said, don't expect a useful information in the first responses... -.-