Article: Vinyl's lower surface noise level than shellac was not forgotten, nor was its durability. In the late 1930s, radio commercials and pre-recorded radio programs being sent to disc jockeys started being stamped in vinyl, so they would not break in the mail. In the mid-1940s, special DJ copies of records started being made of vinyl also, for the same reason. These were all 78 rpm. During and after World War II, when shellac supplies were extremely limited, some 78 rpm records were pressed in vinyl instead of shellac, particularly the six-minute 12-inch (30 cm) 78 rpm records produced by V-Disc for distribution to United States troops in World War II. In the 1940s, radio transcriptions, which were usually on 16-inch records, but sometimes 12-inch, were always made of vinyl, but cut at 33 1⁄3 rpm. Shorter transcriptions were often cut at 78 rpm.

Now answer this question: What was a factor in shellac availability?
World War II