I think part of it is down to El Nino, though I don't think its entirely to blame, you do get odd off years where there isn't much going on. Averages are averages for a reason.From what I understand its down to a mix of El Nino and a ton load of Saharan Dust around in the air, the dust chokes development of storms...and in small amounts is necessary for storm development to act as condensation nuclei. Yet there are 987654321 other factors. No one really knows