There were other internetworking projects at the same time as ARPANET. The DoD's tech was better, and to people like the IETF, "better tech" is all that matters in picking a standard. But most of the people driving the adoption of internetworking among large corporations and organizations weren't DoD people, or even people with a defence mindset. They were just sysadmins, librarians, and communications engineers trying to freely peer their networks with other networks, adopting whatever standards came along that would allow them to do that. Such folk worked on their own standards (see e.g. MIT's Chaosnet) but chose to switch over to the [IETF standardization of the] ARPANET stack when it became clear that's where the future was.