A couple of points to shed some more light on why Telenet, and other wide area networks, used X.25 rather than TCP/IP back in the 1970s.

First, comm lines were relatively slow back then. A 9600 baud line was considered a fast one. 4800 baud was pretty common. So the 5 byte X.25 header looked a lot more attractive than the 40 byte TCP/IP header, just in terms of transmission time.

Second, Telenet was ahead of the game with the X.25 protocol. They were trying to bring up a commercial network before the CCITT actually came out with the first X.25 specification in 1976. My understanding is that they were instrumental in the design of the X.25 protocol.

Third, X.25 was a protocol defined by the CCITT, which was (is) an organization whose members were formally-speaking countries -- because it was a United Nations organization. But, in practice, the members were national telcos. The U.S. was the exception -- I think that the delegates to the CCITT came from ANSI. Maybe someone else knows that for sure. In any case, X.25 had quite a bit of commercial clout behind it due to the interest of all the major telcos.

My faulty recollection is that it was Ethernet that really boosted TCP/IP for commercial use. There the bandwidth made the lengthy protocol headers a non-issue.