What causes a low blood platelet count usually?

Bleeding. Platelets are the scaffoding for clot formation and subsequent tissue remodeling. Platelets aggregate at the bleeding site to prevent further blood loss. Abnormal platelet aggregation/destruction, insufficient production, and certain medication can also decrease platelet count.
Folic acid deficient. Is one of the most common reasons; there are literally hundreds of others. You might start by taking a daily multivitamin, but i it doesn't work you have to dig deeper.