I gave birth in July 2007 and nursed my son until the middle of September 2008. Unlike after I gave birth to my daughter 5 years ago, I did not resume my menstrual cycles as I continued to nurse my son and would only have a few days of spotting.

Now, it has been 2 months since I totally weaned my son, and I no longer even have spotting once a month. What could be the reason my cycles haven't resumed?

Im not sure why scientificly but I know that it just takes time. Every woman is different and you can be different from one child to the next. If you are worried about it call the doctor or a telanurse. They can probably help you over the phone.

As kraussnumber2 said, it can be very individual. You tend to see it more with longer durations of breastfeeding, but it can happen that way for anyone. Some women will start their periods back just from baby sleeping through a couple of feeds at night, it can be enough to start things off again. Other women will find they still have several months without periods after they finish weaning.
Give it a few more months and see what happens. Odds are you will start back up within several weeks. If not, check with your doctor.