Answer:
The market for personal computers is extremely competitive and uncertain, since technological obsolescence happens really fast. For example, during those years better computer chips were continuously launched, even operating systems were launched almost every year, e.g. Windows 98 (1998), Windows 2000 (2000), Windows ME (2000), Windows XP (2001).
Since the life span of computer ships was extremely short, then the risk of signing long-term contracts was extremely high. Once a new chip was launched, the price of older chips decreased severely, so it was better to sign short-term contracts.