Good question, but no meaningful answer. We know what causes wind shear (and also dry air), i.e., an increase in trade winds, a deepening upper level trough, more upper-level lows, but that always leads to the next question: "Why is there more of these or less of those as compared to last season?" The honest answer is "We don't know." Meteorology is still a young science and we have a lot yet to learn about 'why' the atmosphere can make often drastic changes from season to season and/or year to year.

Here are some extracts from the latest CSU updated forecast for the 2006 hurricane season (I have highlighted some of the comments):

"A variety of atmosphere-ocean conditions interact with each other to cause year-to-year and month-to-month hurricane variability. The interactive physical linkages between these many physical parameters and hurricane variability are complicated and cannot be well elucidated to the satisfaction of the typical forecaster making short range (1-5 days) predictions where changes in the momentum fields are the crucial factors. Seasonal and monthly forecasts, unfortunately, must deal with the much more complicated interaction of the energy-moisture fields with the momentum fields.

Reasons for Reduction of the 2006 Hurricane Seasonal Forecast

We have reduced our forecast from our earlier predictions issued in early December, early April and late May. There have been no large changes in any particular atmospheric and oceanic predictor that have caused us to do this. There has, however, been a combination of changes in the ocean/atmosphere system that indicate to us that this season is no longer likely to be as active as our earlier predictions indicated. Physical features which have become less favorable for an active hurricane season are as follows:

1) An increase in sea level pressure values in the tropical Atlantic. Higher sea level pressure values indicate increased stability in the tropical Atlantic which inhibits tropical cyclogenesis.

2) An increase in strength of the trade winds in the tropical Atlantic. Stronger trade winds drive increased evaporation and upwelling which cools Atlantic sea surface temperatures. In addition, stronger trades usually indicate increased vertical wind shear in the tropical Atlantic.

3) A decrease in tropical Atlantic sea surface temperatures. Cooler Atlantic SSTAs (sea surface temperature anomalies) provide less latent heat (i.e., less fuel) for developing tropical cyclones.

4) An increase in Pacific eastern equatorial SSTAs. Sea surface temperatures have still not reached El Niño levels; however, increased warming implies a shift in tropical convection towards the dateline. This eastward-shifted convection often increases vertical wind shear over the tropical Atlantic."

I realize that this doesn't really give you the answer that you were looking for, but if its any comfort at all, many meteorologists are looking for that answer too.
Cheers,
ED