Two main reasons explain the increasing popularity of nineteenth century furniture for collectors.
The first one is related to condition. At the top end of the scale the quality of craftsmanship and materials often surpasses 18th century examples. The 19th century was a period of great excitement and optimism for the future and new skills and techniques were developed to create furniture of exceptional quality and ambition. Because 19th century furniture and works of art were created a hundred years after the 18th century examples many of the very best pieces can still be found in perfect condition.
The second one is directly connected to the 19th century furniture collectors. Often impressive in scale and use of materials, 19th century furniture often proves seductive to art lovers from emerging markets such as Russia or Middle East looking for statement pieces to furnish grand residences. The market makes its rules, because of demand and financial power, and prices go naturally up.