There are few things that get a fanbase as excited as a rookie having a breakout season. Not only does it inspire incredible hope among fans, but getting great production from a rookie is a great advantage for a team. Rookies are relatively inexpensive and cost-controlled for four or five years, giving the team an advantage in cap flexibility.
The flip side of that coin is that there are few things more frustrating than a highly-drafted rookie coming out and having a poor season. Teams need production from their early draft picks as soon as possible, and not getting it can put a lot of stress on the rest of the team and the front office as a whole.