what caused the change from pearl to plastic?

The switch from mother of pearl to plastic was caused by the invention of the plastic button in 1957. The plastic button was easier to produce and less expensive.

Prior to 1957 nearly all buttons were made of mother of pearl (MOP). This shell was collected from the freshwater mussels in the Mississippi river basin, and from many pearling grounds around the world. In fact, the roots of pearl farming in Australia and in French Polynesia can be traced to the MOP button industry.

When MOP fell from favor in the garment industry post 1957, the profitable trade in shell all but ceased. The pearling industry (which only meant mother of pearl in the mid 1900s) would have died and all pearl luggers eventually abandoned if the cultured pearl industry did not develop.

MOP buttons are still being made, however. Many high-end fashion designers still use shell in their clothing designs. In China the local garment industry uses shell buttons on a large scale, and many of the pearl farming regions sell the shell byproduct to button makers or make the buttons themselves.

In every aspect of the fashion industry manufacturers look for the cheapest, fastest, and most cost efficient way to produce goods for garments. In order for you to have clothes that are inexpensive they have to have inexpensive notions (buttons, zippers, etc). And expensive pearls just doesn't cut the cost for many mass production fashion labels or home seamstresses. It's not that it just happened.... but consider it a dollar saved by the maker to have a dollar saved by the consumer.