Though they had considerable resources invested in finishing pianos in varnishes, when lacquer arrived, they began using it, and their output went up immediately. It is for this reason, I believe, that lacquer was so important to instrument manufacturers. Sound quality was probably not a big consideration when compared to the profitability afforded by this fast curing material. Lacquer is a heavily plasticized material. The solids used in lacquer are generally one part cellulose to two parts plasticizers.
Plasticizers are traditionally oils which add flexibility to the film to keep it from breaking.
Cellulose is a very brittle material, and plasticizing it makes for an inherently unstable material because the plasticizers are all unstable substances which deteriorate or leach out after a number of years. Early lacquers used castor oil or camphor as plasticizers but as time went by the list of materials grew too long to begin to list all the different substances used for this purpose. Some synthetic materials are even used to try and make the films more long lasting. The history of old nitrocellulose film is a good example of the lack of longevity of this kind of material this is why there is a film restoration business. Likewise, the problems of finishes cracking and their poor adhesion on guitars can also be traced to this phenomena.