Why do people use Logical Fallacy?

The use of a logical fallacy may occur for different reasons.

A: The debater lacks intelligence at least in the realm of legitimate debate.

B: The debater doesn't have much background knowledge on the topic of discussion

C: The debater is not interested in persuasion or the exhanging of ideas but in demeaning the listener and/or individual(s) who are a part of the topic of debate

There may indeed be more reasons, but these are the ones I have personally encountered.