The term "conspiracy theory" is a label that is usually stamped on inconvenient allegations of dishonesty or fraud by government.
Entirely reasonable questioning can be ridiculed by labeling it a conspiracy theory. In the popular imagination the word conspiracy theory has been given a connotation of paranoid delusion. This has been done by deliberately circulating implausible allegations, giving them press and then shooting them down, by character assassination attacks against people asking inconvenient questions, by intimidation and by a host of other manipulation techniques. Nobody consequently wants to be called a conspiracy theorist. The term is almost synonymous with crank.
The term conspiracy simply means a secret agreement to cooperate on committing a crime.
Any business or government makes secret agreements all the time. This is an implicit aspect of the society and mode of existence we live in. That some of these agreements would concern criminal or unethical activity is to be expected, specially when one has little chance of ever being made accountable.
Thus it becomes all the more necessary for those who actually engage in such activity to promote the greatest conspiracy theory of them all, i.e. the fanciful imagining that there exist no conspiracies.