The Golden Dawn's achievement was curricular. It took material scattered across Renaissance magic, Jewish mysticism, astrology, alchemy, tarot and the Enochian records of Dee and Kelley, and arranged it as a course of study with examinations. The Outer Order ran from Neophyte through Zelator, Theoricus, Practicus and Philosophus, each attached to a sephirah and an element, with written tests before advancement. A Portal grade led to the Second Order, where the practical work began.
The syllabus is why the order matters more than its size. A member learned Hebrew letters and their correspondences, the tattva symbols, geomantic figures, the tarot as a system rather than a fortune-telling deck, ritual construction, and scrying. Almost every Western magical order since teaches some version of that curriculum, usually without knowing where it came from.
The collapse was fast. Mathers, running the Second Order from Paris, was challenged over his authority and over the Sprengel correspondence; Crowley's attempt to claim initiation on Mathers' behalf brought matters to a head in London in 1900; and the order split into the Stella Matutina, the Alpha et Omega and other bodies within a few years. That the material survived at all is largely due to Regardie publishing it in the late 1930s, against his obligation, on the argument that it would otherwise be lost.