Why are the Perfection of Wisdom sutras seen as a coherent collection?

The Diamond Sutra , Heart Sutra , the Perfection of Wisdom Sutra in 8000 lines and others are grouped together as Prajnaparamita (perfection of wisdom). Why is this? Why are they seen as a coherent collection? Are they thematically similar or is the link because they were written at a particular geographic and historical period? Are they maybe linked because of they a reaction against other schools of Buddhist thought at that time? Should they be linked together at all?