Society increasingly relies on the Internet as a critical infrastructure. Inter-domain routing is a core Internet protocol that enables traffic to flow globally across independent networks. Concerns about Internet infrastructure security have prompted policymakers to promote stronger routing security and the Resource Public Key Infrastructure (RPKI) in particular. RPKI is a cryptographic framework to secure routing that was standardized in 2012. In 2024, almost 50% of routed IP address blocks are still not covered by RPKI certificates. It is unclear what barriers are preventing networks from adopting RPKI. This paper investigates networks with low RPKI adoption to understand where and why adoption is low or non-existent. We find that networks’ geographical area of service, size, business category and complexity of address space delegation impact RPKI adoption. Our analysis may help direct policymakers’ efforts to promote RPKI adoption and improve the state of routing security.