Lots of reasons. Here are a few I've noted offhand.

1. How relatable is the issue?
2. How heartwrenching?
3. How easy to read? (Length and complexity both factor in.)
4. How familiar is the poster with the community?
5. When is it posted? (How busy is the forum at the time?)
6. How open is the poster to replies?
7. How engaging? (For example, it's easy to garner replies to polls where respondents can just vote and/or make a quick comment.)
8. How much does the poster seem to need help and how easy is it to give? (i.e. complex or simple issues, etc.)