Only speaking for myself here, but the reason I first started looking into the pre-2000 games was pretty much D&D 4th Edition. .
For me it started mid 3E with frustration over playes increasingly expecting adventures structured around encounters and the CR system and 4E was the final thing that caused me to go back to older editions.
i was getting increasingly frustrated with how adventures were structured in places like Dungeon and how people were coming to expect them to be built around planned encounters. I picked up the 1E DMG again, around 2005, which i hadnt really read through because i was more of a 2E GM, and found a lot there i liked in terms of adventure design. Wasn't 100% on board with everything, but it spoke to me more than what i was seeing in dragon and what wotc was putting out, and it reminded me of my early days when we just kind of allowed things to happen and focused on exploration. So for me reading the 1E DMG was a big part of it, but i still continued playing 3E. Then when 4E came out, i bought the books and realized it wasnt for me. The style of play (at least how it seemed to be presented) and mechanics just didn't appeal to me.
so i started playing other systems more, and during that time, i decided to run a 2E ravenloft game, because there is so much good ravenloft material written for the system. Once we played, it occured to me he system made a huge difference. My ravenloft games had felt stale under 3E, and i just figured it was because i was older and the setting wasnt as shiny, but using 2E, i felt inspired again and the flow of th game just worked better somehow (a big part of it for me was not having things like diplomacy and spot). Soon after one of my friends showed me lamentations of the flame princess, and then i started looking for old 1E material and picking up some of the retro stuff.
I am by no means a strict OSR gamer. I play 1E, 2E and various retroclones (including one we published) but I also still play 3E, Network System, Savage Worlds, and look into newer games like numenera (which i am eager to try).
One thing that surprised me going back to AD&D is i had assumed it would feel very clunky agter not playing it for a decade of 3E, but the system was not the problem i thought it would be. I realized while 3E did smooth a lot of stuff out, it also removed things in the process or added things that changed the exprience of the game. One good thing about OSR material is its the best of both worlds, they smooth things out, or keep 3E innovations that are popular, but also bring back some of the elements that are missing from current editions.