I think a lot of it has to do with the time that someone can commit each week. Naturally, those who can commit more time each week to the quest will finish sooner. From AOPA's website:
"How long does it take to learn to fly and get a Private Pilot certificate? (top)
The same variables that affect the cost of learning to fly (frequency of flight lessons, weather conditions, the kind of aircraft in which you are training and its availability for scheduling, and individual aptitude) affect the time it takes to earn your certificate. The FAA has a minimum number of flight hours needed to obtain a certificate. Under Part 61, the minimum is 30 hours for a recreational certificate and 40 hours for a private pilot certificate. Some schools operate under an alternate regulation, Part 141. These schools have more FAA oversight, more rigid schedules, and more paperwork. For the added requirements, they are allowed to reduce the minimum hours of private pilot training to 35 hours. Many schools estimate that a true average flight training time for a private pilot is between 50 hours and 60 hours for both Part 61 and 141 schools. Others believe that 68 to 70 hours is the more likely average. These flight hours can be spread over a time span of several months or a year or more."
I think another contributer to this additional time beyond the minimum required is likely advanced cockpits we see in "primary" trainers today. It's rare to find a newer 172 without GPS, and many of the SPs will likely have an MFD and auto pilot in addition. In some cases learning the buttons in the aircraft is almost as difficult as learning the flying part itself.
The stuff from AOPA was taken from this link:
http://flighttraining.aopa.org/learntofly/faqs/#5 you can probably find answers to other questions you may have there as well.