Say what? Demonstrably not true. 2e, in it's entire run, did not outsell 1e in it's first 3 years. Or even first year. 2e chugged along because it was a band aid through poor business decisions and bad communication between departments. Ben Riggs lays it out in his book. Initial sales were high but not like 1e sales and then tanked. Products would sell for a couple months and then bottom out.
4e only outsold 3e in initial pre-orders and while it was still a success it wasn't a Hasbro level success and was dead within 3 years of release.
3.5 lost gas after about 3 years on the market because the OSR was born and started the market split and the release of 4e caused an open wound in the market that Pathfinder bandaged up by being the new edition that 3.x players were wanting when 4e came out.
That 5e has been the massive success it has been is a shock. They had essentially no budget, no team and very little backing from WOTC/Hasbro when it came out. The D&D team was on fumes compared to the past. Mearls, Crawford and Perkins caught lightning in a bottle and synergized, unknowingly, with an unsuspecting pop culture thanks to Stranger Things referencing D&D so much in season 1, Critical Role becoming a massive success, none of that was expected to happen and contribute to D&D business in a big way.
So no, each successive edition did not outsell the previous edition. 3.x didn't even outsell 1e or basic, it's measure of success was 2e that's how successful 1e and the original Basic set actually were.