It's just the way their separate species developed over time. Pythons are oviparous and boas are ovoviviparous, but they are still very much cold-blooded reptiles. Even though boas give "birth" to live young, the babies are still incubated inside of eggs within the mother's body. Unlike most mammals, boa babies are not connected to a placenta and are instead sustained by an egg yolk. It must have been an evolutionary advantage for boas to develop like that, because instead of having to lay the eggs and risk them being eaten or susceptible to the environment, they are kept safe within the mother's body until they hatch.