Because till now in Barca's shirt, ALL Dembele's good games were in matches where he had lots of space for counters and for running with a ball in his feet for 30-40 meters and dribbling through defenders.
Whenever an opponent parked a bus, Dembele more or less hadn't create anything till now.
Basically, for now, against parked buses:
1. if he gets the ball around the box on the right side: he can try to run a few meters and cross. But since we have short players, crosses aren't a good idea
Or, he can do the same and play a backpass to the edge of a box for Messi, which is predictable.
2. he can receive the ball (without running for a few meters) and just pass it back in the middle to the edge of the box where Messi or Iniesta always wait for the ball. Predictable.
3. or, the best case, he can try to dribble through a parked bus or try a 1-2 AGAIN with someone at the edge of a box, which is predictable and easy to intercept
So, basically, when we play against parked buses, the only "unpredictable" and useful thing from him (for now) is if he'll try to dribble through 3-4 defenders in a parked bus.
Our game in general is too predictable, one-dimensional, you need to pass to Messi in every action, we can't cross, we can't play counters with a possession sytle.
Plus Dembele is a counterattacking player for teams like BVB.
For example, yesterday against a parked bus:
Rakitic is not good against parked buses.
Iniesta is past it.
Semedo can't do anything against parked buses.
Dembele can't do too much against parked buses.
Alba is lost without Messi.
Luis can't do anything regardless if they park a bus or not.
So, basically we only had Busquets who can make a good pass here and there and Coutinho, the only guy suitable to play against parked buses and who has weapons for that style of play (OTHER than passing to Messi in each action).