It's not just length; several other heuristics contribute to a question's "quality score", and if it scores too low, we block it. Usually, this works out well - questions that fail too many checks are in dire need of cleanup, and giving the author a chance to do this before posting saves everyone time and grief.
However, there are edge-cases where the system falls on its face. If you can't get around the problem by doing a bit of cleanup on your text, you can always ask us to take a look at it.
And yes, the message could be more specific, but that would defeat the purpose - we don't want folks to just pad out their questions until they get through, we're hoping they put a bit more thought into what they're asking.