according to Guide to Literary Agents
1. Generic beginnings: Stories that opened with the date or the weather didn’t really inspire interest. According to Harmsworth, you are only allowed to start with the weather if you’re writing a book about meteorologists. Otherwise, pick something more creative.
2. Slow beginnings: Some manuscripts started with too much pedestrian detail (characters washing dishes, etc) or unnecessary background information.
The other 5 reasons are here.