If you are talking about the workout, it's because my front rack sucks. It's just been in the past month that I've felt comfortable with any sort of weight in the front rack. The 105# I did for the workout was hard enough considering every thing else. I'd guess the RX'd weight of 145# would be close to my 1-3 rep max.
For just training the movement that makes sense. I'm a garage crossfitter and I have a neck issue that precludes my being able to overhead squat, but I can front squat with no problem. I front squatted 225lb about a year ago, which is typically about all my right knee will allow me to low-bar back squat.
Since I can't do overhead squats that precludes my being able to do heavy snatches so I just clean and jerk, power clean, and power snatch. Rippetoe is of the opinion that that is all that is required for general athleticism and GPP.
