Because I don't feel like it? I have BiS gear both for tanking and DPS on my powertech, but I find tanking with her to be boring since the tanking style is too passive so I only use the tank stuff for guild raids/runs. I've tanked everything in the game with her, so skill isn't the problem, but I just don't like it.
And yet you ignored my next line lol. I know DPS on a PT is awesome! DPS on my Guardian makes me never want to tank on any FPs or HMs. But its a necessary evil when it takes me 20 minutes to queue for a FP and longer for Ops.