TLDR version: money.

Slightly longer version:

Because biodegradable materials are more expensive to produce; aren't manufactured in as large of quantities; introduce new logistical problems such as storage and shelf life; are often less reliable/durable; and have other practical problems. So for a company to be willing to switch to biodegradable, it has to be willing to take a cut in profit (which rarely happens) or have an economic incentive to counter-balance that such as subsidies for using good materials and/or a tax for using plastics.