1. It's what everyone knows and uses. If you say "demon", it may take most people a few seconds to realize what you mean; "day-mon" is instantly understandable by all software people. Etymological purism should always take a back seat to clarifying thought.
2. "Day-mon" has better connotations among people unfamiliar with the concept. We don't need tech-illiterate people thinking there are demons in their computers, whereas a "day-mon" sounds like something benevolent and helpful (especially among younger people who might've read The Golden Compass books).