Why would he need a big increase in catches to be more involved in the passing game? The type of patterns he was asked to run was more indicative of Wallace also becoming a chain mover than just a "one-trick pony". The Steelers offense under Bruce Arians took advantage of Wallace's big play ability while the offense under Todd Haley did not. Haley wanted Wallace to run shorter, quicker, timing based routes in order for Big Ben to get the ball out faster and it took awhile for them to buy into that concept.
Because shorter routes are more common than deep routes and are completed at a higher rate. Therefore Wallace should have had more catches.
What exactly does "more involved in the passing game" mean for a WR if not more catches? They tried to get Wallace more involved but weren't very effective at it? One trick pony.