Reed was used well before 1900 as roofing. It is a natural material that often grows in moist areas. A thatched roof has the advantage that it used to be relatively cheap and, incidentally, also insulated relatively well. The material grew in many places the Netherlands, and the labor cost for roofing were still low.
Nowadays, having a thatched roof is kind of astatus symbol and certainly not cheap anymore. Reed is very little cultivated, labor costs have risen sharply and fire insurance has become a major cost.