well for one: there's more data recorded in the flight data and voice recorders then there is being transmitted in the occasional maintenance ACARS message.
Maybe if bandwidth is unlimited and services free, it is no problem. As long as that's not the case, it is out of reach. Just think of how mana flights are operated worldwide at this very moment; then you can figure how much data this is generating. By the way; not all aircraft have a datalink system.