1) The content in the OP could be wrong.
2) The content in the OP may not have been well structured, thus making it ambiguous to read/understand.
3) The content in the OP was just a ploy at fishing merits.
4) The content in the OP was redundant.
5) The content in the OP helps absolutely no user here.
6) Users/merit sources who saw the post didn't feel it deserved any merit, or it was a topic they are not interested in, etc, etc.
Replies to a thread could earn merits for quite a lot of reasons, a few of which are:
1) The reply corrects the misconception of the OP.
2) The reply was better structured and sensible than the OP.
3)The reply contained some new/novel information that can help other users.
4) Other users or MS considered it worthy of merits.