I hate conspiracy theories too, but it's important to realize that there are conspiracies in the real world. After all, the CIA is one big conspiracy. However,
1) no Conspiratorial Elite can pull off its fiendish Plan without a lot of help from followers. It has to communicate with them and motivate them to go along with the Plan. Thus, it is quite likely that information about the Plan will leak out before and after it's pulled off. It's likely that different parts of the same organization will work at cross-purposes, while some people will defect. It's extremely unlikely that the Plan will work perfectly.
Most groups stick together partly due to a shared ideology. But any ideology is subject to interpretation by different individuals. Many might see the Conspiratorial Elite's Plan as going against the "true" meaning of the ideology.
(Think of how hard it is for the left to get its shite together. Does the right have an easier time?)
2) the Conspiratorial Elite itself lacks complete information about what's going on, what to do to attain its goals, and how individual followers should behave at each stage. One problem is that the Elite may believe its own propaganda.
3) there are competing organizations and elites. Often they don't get along. This weakens the ability of a Conspiratorial Elite to attain its goals.
I think it's rare that an Elite actually attains its goals; al Qaeda's 911 action seems the exception, not the rule. -- Jim Devine "Segui il tuo corso, e lascia dir le genti." (Go your own way and let people talk.) -- Karl, paraphrasing Dante.