HAPS are fragile due to weight constaints, e.g. NASA’s Helios. Biplanes are more robust.
Although biplanes have high drag, HAPS only need to move slowly because at their operating height the wind speed is less than 50 mph, and to maintain station they only need to overcome that.
A biplane is more robust than a monoplane per unit weight. This is because the monoplane supporting beam has to live inside the max vertical depth of the wing. With a biplane the beam can be as deep as the vertical distance between the upper and lower wing. The biplane has a worse lift to drag ratio than equiv monoplane. But drag increases dramatically with air speed. As the HAPS is only slow flying maybe the drag is acceptable. At high altitude the Reynolds number decreases, this raises stall speed. Stall speed goes down with lower wing loading (plane weight / wing area). Biplane wing loading is lower so will have a lower stall speed. Of course it's all trade offs. These are arguments for the biplane.