Why are they non renewable?

Answer 1

Peat is formed from the roots and stems of dead plants, primarily sphagnum moss, in large marsh and swamp areas over the course of hundreds or thousands of years. As each succeeding generation of plants die, they leave their remains. Because of the anaerobic conditions these remains can not fully decay, and thus accumulate, layer after layer.

There is debate as to whether or not peat is actually non-renewable.

Peat forms at a rate of about a millimeter a year. As such, it can take over 300 years to replace a foot of harvested peat. Because of the huge abundance worldwide of peat bogs, this is not as big of an issue as it may seem.

However, a perhaps greater issue with the use of peat is the method by which it is harvested. Peat bogs are very biologically diverse areas, home to a huge number of animal and plant species. When peat is harvested, drainage channels are cut into the bog, drying it out, and essentially killing the entire ecosystem. Although laws exist requiring harvesters to restore the wetlands to their previous condition, doing so does not fully restore the habitat. Once the area is killed, it can take hundreds of years, if ever, for the ecosystem to restore itself.

Previous answer:

(Peat is non-renewable because it is a type of rock/mineral which can take eons to grow back, classifying this as non-renewable.)