His defense in the outfield is extremely terrible. If you just look at 1B then Smoak is still better defensively but it's not a huge gap anymore.
Batting wise, Colabello has wRC+ of 146 compared with 96 of Smoak.
Value wise, Colabello has 0.7 WAR at 304 PA compared with 0.1 WAR at 258 PA.
I guess they want to keep them both and make sure they are fresh.