In retrospect, it was kind of a lateral move for the Steelers, except that Fields had put the ball on the carpet six times in six starts. Wilson fumbled five times in 11 starts.
It's true that Fields had only thrown five TDs through his six starts ... but he also rushed for five TDs, as well. That meant that Fields accounted for 10 TDs in six games -- 1.67 per game. To be fair to both Wilson and Fields, that is just barely about league average (maybe a bit below). Wilson himself had 18 TDs in 11 starts (1.64 per game), so they were almost identical on that particular stat.
It was kind of a weird season for Steelers' QBs. Fields started the season's first six games, starting off 3-0 but then losing two straight games by field goals (@ IND, vs DAL). The next game, Fields and the Steeler drub the Raiders on the road by 19. Tomlin then makes the switch to Wilson (the draft-pick motive you mentioned). Wilson starts off like a house afire, going 4-0 including nip-tuck wins at Washington and hosting Baltimore. Wilson goes 6-1 over his first seven starts, then the bottom drops out -- the Steelers lose their last four plus their playoff game without scoring over 17 points in any contest. Three of Wilson's five fumbles came during the season-ending slide.