Who knows? Maybe nothing. Until their little muscles are ready to make it through the night, or send the signal to the brain and wake them, it happens.
My oldest JUST started staying dry through the night and she is almost 9. My son has been dry through the night since he was 3. When I have asked our pediatrician about it, he told me not to worry about it and so I chose not to. I opted NOT to wake her during the night and take her to the bathroom, nor did I use the alarm system that I purchased and intended to use. My pediatrician said the alarm would be of little help unless the child was motivated. She was definitely not motivated. There is medication out there, but since it works out for many at an older age, I opted not too. Plus, I am not big on meds for situations that are inconvenient.
My daughter is a very sound sleeper and I think that was part of the problem. She even asked the other day why all of a sudden she doesn't need a pullup at night. My answer was simply that every child is different and her body chose sleep over getting up to go to the bathroom. My advice would be to put the pullups on and treat it like no big deal. Just my 2 cents.