I'd offer that it's due to the speed of the plane. I have seen it done with mono-wing planes as well, but it's rare, as others have also mentioned. The rationale for biplanes is likely twofold:

1. There's a nostalgia element to be considered, now that the style of plane is out of fashion, and has been for some time.
2. Two wings means more aeronautical lift. The additional weight in the plane or just the stress on the plane itself combined with lower airspeed means that the plane needs all the help it can get to remain airborne, especially when stunts are involved.

I'd offer that it's due to the speed of the plane. I have seen it done with mono-wing planes as well, but it's rare, as others have also mentioned. The rationale for biplanes is likely twofold:

1. There's a nostalgia element to be considered, now that the style of plane is out of fashion, and has been for some time.
2. Two wings means more aerodynamic lift. The additional weight in the plane or just the stress on the plane itself combined with lower airspeed means that the plane needs all the help it can get to remain airborne, especially when stunts are involved.