Mendel choose pea plants for studying inheritance because pea plants had a no. of clear-cut differences which were easy to tell apart. In simple words, Mendel chooses a pea plant to study inheritance because many generations of pea plants can be produced in a comparatively short time span and their study is much simpler than that of animals.
For example, some pea plants were tall while others were dwarf, some pea plants produced round yellow seeds whereas others produced wrinkled green seeds, etc.