EAGLE gained massive popularity because it provided powerful, professional-grade capabilities at a fraction of the cost of some other commercial tools, while also being cross-platform, supporting Windows, macOS, and Linux.
Despite its age, version 6.5.0 is still discussed in the engineering community for several reasons: Eagle 6.5.0 bNames import issue (text rotation) - Software eagle 6.5.0
Eagle 5.12 Linux on Ubuntu Saucy 64bit? - element14 Community while also being cross-platform
Many bugs found in version 6, such as slow QuickRoute performance or persistent group deletion errors, were eventually addressed in the EAGLE v7 updates End of Life: and Linux. Despite its age