The pre-service training of primary and post-primary teachers differs because of the differences between the primary and post-primary systems of education.
Primary education requires teachers to be generalists: able to teach all the subjects of the curriculum. Their methodology must be also adapted to the age of the pupils.
Second level teachers are not generalists: they teach only one or perhaps sometimes two or three subjects depending on the size of the school. Their methodology is subject-specific and must take account of the fact that their pupils are moving through perhaps the most difficult period of their development.