IME, it's an edition thing, in addition to a cultural thing. OD&D was rules light and meant to be run and modified by the DM (there's even a suggestion on how to make a dragon PC). 1E was intended to be truly RAW, a unified system out of the hodgepodge of OD&D, but it was such a mess that houserules were necessary to have a reasonable game. 2E was much more friendly with homebrew and houserules, many of the popular ones becoming staples in 3E.
3E and 4E became the era of RAW, and this is where culture comes into play. The game didn't encourage houserules or homebrew (despite the fact that all 3PP are essentially homebrew), but it wasn't just D&D effected by this mindset. I found other games being pushed in a similar direction, and I think it was largely due to the frequency of internet commentary. Without an agreed upon set of rules, it made discussion and comparisons difficult, so the idea of RAW became entrenched. I tried RAW for quite some time, but I found that it limited me as a DM and player to just what the game specifically allowed.
5E is the best system for houserules and homebrew, because it's built upon a solid chassis. You can make a lot of tweaks, while still keeping it "D&D," but you can go whole hog and turn into something almost completely different using the same base mechanics. By abandoning RAW, this frees up the DM to work with their group to customize the game for their specific needs. The game was playtested to appeal to the largest possible base, but a compromised edition seldom works for most groups, while now everyone can make it work for them.