There are numerous scientific hypotheses as to why people fail to remember their dreams.
1. Interference. Upon waking, many people immediately begin to run through their morning routine. These recollections far exceed the importance of the dream one just had. The morning routine is given precedence over the dream and the dream is forgotten (in the brain's rush to get the day started).
2. Some dreams are quite taboo (by culture and personal preference). Therefore, people make the unconscious decision to forget the dream because it goes against the person’s individual morals.
3. Some scientists believe that "we dream to forget." Essentially, dreams are the snippets of things our brains "trapped" throughout the day, but did not process (information the brain did not deem important, did not move to the long-term). What this is saying is that our brains are triggered at times when we are unaware. These triggers tend to be little pieces of information which "hitchhike" with one throughout the day. When one dreams, the "hitchhikers" are disposed of.
4. Dreams are not processed in the same way which active information is processed. Essentially, one's brain fails to store the dream effectively and this causes the dream to be forgotten.