Despite the fact that 1E had charts and tables for some classes extending quite far above 20th (and Basic even went wildly further) there's a reason that some classes were given absolute maximum levels even lower than 20th in 1E - it wasn't particularly expected that players would WANT to, much less be ABLE to advance characters that far. Many/most 1E classes were being given name/title level benefits because it was anticipated that players would be retiring or at least semi-retiring those PC's and looking for new characters to play. NEW class special ability gains were fast drying up at that point and advancement meant little more to non-casters than a FEW hit points, plus to-hit/save improvements. Casters still gained more spells and higher spell levels but again it was still largely thought by GYGAX in 1E that PC's would be getting retired by players voluntarily, not that they'd keep playing them until they literally couldn't be resurrected anymore.
By the time it gets to 20th level AD&D has become a hugely different game from where it begins at 1st level and is not holding together well. Anybody wanting to take it beyond that point is still perfectly free to do so - it just really isn't worth lending a lot of official support to that kind of game. Don't know if that's actually the reason, but it's always been my personal perspective. While SOME people still embrace very high level play, surveys have always repeatedly shown that the majority of play (whether by deliberate choice or circumstance) is concentrated below double-digit levels or falling off when reaching that point. That almost certainly wasn't definitively known at the time 2E was done, it was still not hard to sense that. In the Strategic Review Gygax had noted that AT THAT TIME the campaigns for Greyhawk and Blackmoor were both about 4-5 years old and yet no PC had advanced yet above 14th, and it was strange to him that even playing 50-75 games in a year that anybody would have a PC higher than 9th-11th level. AD&D was really not designed nor anticipated to have such high-level games, even if SOME charts and tables extended that high (and really would be more likely applicable to select NPC's, rather than seeing a lot (any?) use for PC's.