Because plants cannot use the organic forms of nitrogen that are in the soil as a result of:
(1) wastes (manure and sewage)
(2)  compost and decomposing roots and leaves

How does ammonification occur?

Microorganisms convert the organic nitrogen to ammonium.  The ammonium is either taken up by the plants (only in a few types of plants) or is absorbed into the soil particles.  Ammonium (NH4) in the soil is stored up to later be changed into inorganic nitrogen, the kind of nitrogen that most plants can use.