I have no data to back up my personal hypothesis...
The Brewers lineup is usually pretty "deep" in the sense that even most of the poor hitters take professional ABs (Perkins, Collins from last season, etc.). They are pros enough to make the pitchers execute in multiple ways, which is hard to do consistently. Ortiz isn't one of these guys, for whatever reason. So, when opposing pitchers fail to execute, or even choose to pitch around a guy like Perkins, they can relax and just pitch to K, which most pitchers are comfortable trying to do. Or even pitch to contact with 2 down, because compared to the #8 hitter, Ortiz just isn't much of a threat.
Against Ortiz, pitchers can just use their best one or two pitches over-and-over because Ortiz is EASY to execute against. He is always a battery's plan B, which they are pretty comfortable using.
So basically, my hypothesis is that the Brewers' #8 hitters just take really professional ABs.
edit to p.s. - So basically, Ortiz is the least hitter in the lineup. Brilliant analysis.