It comes from the fact that the windows have a metallic content to prevent solar gain in the sunshine (it helps keep the air conditioning load down in the summer). Of course the rest of the carriage is effectively a Faraday cage, preventing microwaves from leaving the carriage.
When Virgin wanted to install signal boosters, the only mobile company who agreed to working with them was Orange. Once the electric trains (Pendolinos) came to be fitted, all of the operators were willing to co-operate so the internal boosters handle all operators.
Most of the Voyager units were passed over to Arriva when they won the Cross country franchise, and I don’t belive they’ve done anything with them since (possibly even maintain the boosters that were installed for Orange mobiles)