Why do I have to put at least x number of characters in the title?
You might have noticed that you are asked to "be descriptive" when writing the title. The purpose of that is to have high chances to find a certain question using relevant keywords. Since the title always has a high rank in any kind of search (site-internal, google, or other), it should be a good (even if concise) description of the question.
This kind of descriptiveness is (supposed to be) encouraged by the minimum character count.