They own the infrastructure and at the end of the day their techies have the most experience.
Actually, when France Télécom took a controlling share in TPSA, after the initial inspectorate report of the exchanges and computer systems, France Télécom was surprised because it was more modern than their own system in France.
This was because the whole system was modernised after the fall of communism in Poland, although the line infrastructure needs modernisation the basic computer system is one of the best in Europe.