Marketing maybe? If you are good about being backward compatible, you never release n+1, which might be mistaken for being stale vs your competitors.
If I worked on a product that was version 2.1 when I started, and 5 years later was on version 6.5 without any actual rewrites or improvements in it, I would find that just as demotivating.
Such a stable environment would leave a lot of room for side projects on a stable paycheck, or getting some extracurricular qualifications.
Edit: added Vista... thanks!