Dickerson developed beyond what the Pirates and most people saw in him, while Decker just stopped developing. Decker was a former sandwich round pick Top 100 prospect who showed more power in the low minors (seasons of 16, 17 (in half a season), and 19) to go along with decent wheels, a solid glove, and a great eye...meanwhile Dickerson looked like he was going to fill out as a AAA caliber pure 1B without the power to break through to the Majors. From our POV it was trading a guy with a higher (but still minor league) floor and a limited ceiling for a guy with a low floor and a high ceiling...it wasn't a head-scratcher of a trade at the time and I think most of us would advocate deals like that more often than not.
105 at bats is a minuscule sample size. Remember when Chris Shelton hit 9 home runs in the first two weeks of the season about a dozen years back? He fell off the map almost as quickly.
Dickerson would never have gotten the 'value' he has now, because he'd be blocked not just in Pittsburgh, but in Indianapolis. The Padres also got extra value out of him by turning a guy that profiled as a pure 1B into an outfielder...but that would have never have happened in Pittsburgh because he isn't athletic enough to play in our outfield (save your snarky McCutchen comments, McCutchen was ungodly athletic when he came up). Any time you trade a prospect for a prospect you risk this happening...but who knows if Dickerson will even keep this up once a book is written about him.
And things change...at the time of the deal 4th outfielder was a far bigger near-future need than 1B.