Why is PPF curved concave?

PPF is concave to the origin because MRT which is the slope of PPF goes on increasing from left to right because the resources are inefficient in production of both the goods. When the resources are transformed from the production of good y to good x, the resources which were suitable for good y may not be suitable for good x and hence they produce lesser output of good x. As a result, the opportunity cost of producing good x goes on rising from left to right on the PPF. Hence, PPF is concave to the origin.