The answer is…
To hide their trade secrets!
The origins of glassmaking in Venice go back to the times of the Roman Empire when molded glass was used for illumination in bathhouses. Blending Roman experience with the skills learned from the Byzantine Empire and trade with the Orient, Venice emerged as a prominent glass-manufacturing center as early as the 8th century. One of the earliest furnaces for glass on a Venetian island, dating from the 8th century, was discovered by archaeologists in 1960.
By the late 1200s, the production of glass objects of the finest quality was the city’s major industry as confirmed by the establishment of the Glassmakers Guild that laid out rules and regulations for the craftsmen. The purpose of the guild was to safeguard the secrets of the trade and ensure the profitability of the industry. In line with these objectives, a 1271 law prohibited the importation of foreign glass or the employment of foreign glassworkers.
An even more radical law was passed in 1291 that laid the ground for the establishment of Murano as a premier glass-manufacturing center. This law required that all furnaces used for glassmaking be moved from Venice to Murano to avoid the risk of fire from the furnaces spreading onto the largely wooden structures of overpopulated Venice. Many historians agree that the true motive for this law was to isolate the glass craftsmen to a location where they wouldn’t be able to disclose trade secrets. A subsequent law passed in 1295 forbidding the glassmakers from leaving the city confirms this theory.