While Apple hasn't really been dragging its heels, it hasn't been champing at the bit either. The wait for OS X has been even longer. These extended cycles can have a negative impact on the company's image. Granted, when Apple delivers, Mac users usually get far more than they expected. Unfortunately, the long wait can tarnish the final product.
Investors are also a skittish bunch.
Long pauses between the introduction of new products makes some people nervous. Announcements at more regular intervals might help Apple in the long run. Mac users would also get to scratch their restless itch more often. Rather than a massive blowout a couple of time a year, we'd be continuously distracted by new things from Apple throughout the year. Of course, there a only so many things that Apple can produce.