This question already has an answer here:
This question is similar to: Why there isn't reputation points for successful flagging? , but with a small refinement,
- only first flag will get +2 reputation
- only accepted and committed flag will get the reward (this is: question got closed as duplicate, for example)
- only 'exact duplicate' and 'offtopic -> move to other sub-site..' flags will be eligible for bonus.
- there is a high volume of newbie questions appearing each day, which are already answered, sometimes answered at epic level.
- because answering such question can give a person +25 reputation from the noob, not counting random by-passers' good will - people are not very willing to mark such questions as duplicate, and sometimes deliberately copy-paste answers from older duplicates.
- offensive, spammy or retarded questions are downvoted with passion anyway, so there is no need to additionally reinforce users to do that ;)