Apollo Explorer, a super-charged version of GraphiQL, is not open source because it requires significant resources to maintain and evolve. The creators of Apollo Client and Server have learned from past experiences with open-sourcing other projects, realizing that maintaining popularity comes with increased responsibility and work. To address this, they employ multiple teams of engineers to work on their core open-source projects full-time, making a commitment to the health and betterment of these projects without direct revenue gain. Instead, Apollo Explorer is made available through a CDN, allowing users to embed it in their websites or developer portals with customization options, while maintaining quality updates and rollouts.