They need to be able to get themselves to the toilet. Emotionally, it is important for two year olds to feel ready to make their own decision to use the potty. Further, some children like to hold on to their diapers because they offer the security and familiarity of babyhood. If they are pushing for independence in other realms, they may be reluctant to move out of their diapers for awhile. The other part of emotional readiness has to do with fear.
Some children are afraid of the toilet.
Many children successfully learn to pee in the toilet, but take much longer developing the sense of security and confidence needed to let go of their poop without a diaper. Cognitively, children need to be able to understand that pee and poop go in the toilet. They need to be able to interpret the physical sensations that tell them that their bladder and bowels are full. Alice can't concentrate on digging a hole in the sand at the same time she s thinking about whether she needs to pee. Bennie can't be expected to think about the poop that may be coming if he is studying an ant colony disappear into a hole in the ground.