Probably because they want cameras they know will work and survive in a deep-space environment. Space is hard. Once you get out of the atmosphere and magnetosphere, it's an unkind environment for electronics. They're probably going to spend the money, effort, and mass on some nice cameras for manned missions, but these are little cameras that mount to the ends of the solar arrays.
It is also probably a data rate issue. They will have higher priority data coming back from the spacecraft, especially for a first iteration, and in the early stages of the mission.
It is quite possible that once things have stabilised that the video and pictures being sent back will improve. For the ispace lander we take much lower resolution images throughout the mission until we have landed and have the high gain antenna in a stable connection.
