Although compostable and biodegradable are often used interchangeably, they do not mean the same thing. Biodegradable means that a product can break down into natural materials in the environment without causing harm. The time frame can be months to years. Compostable, on the other hand, means that a product can break down into natural materials in about 90 days.
There are many different compostable plastics on the market and the answer would be different for each one. The short answer is: because what looks like plastic is treated like plastic. Compostable plastics only compost in commercial facilities. Unfortunately, commercial composters sort out every item that looks like plastic. That means if it looks like plastic, the cutlery will end up in the landfill where it can take hundreds of years to degrade.