Science is always looking for the reason why something is the way it is. For now we only know that the energy that controls space is the same energy that controls time, so they trade off, the faster you go the less energy there is for time to move. It's why science now uses the term space-time.

"Why" is still technically unanswered, but you can always go another "why" deeper anyway and get a new question, it's what's so fascinating about science!