Why do most minecraft servers run on 1.12.2?
TStodden: SHORT ANSWER
SHORT ANSWER: Depends on their configuration & the current size of the world. LONG ANSWER: Some servers are running on a modded platform (like Bukkit, Spigot, Forge, ect.) & potentially mods that haven't been updated to the latest version... which is TYPICAL for non-vanilla servers as there's no official modding API's from Mojang. It can generally takes a few weeks for the modded server platforms to release to the most recent version. Then it can potentially take MONTHS for mod plugins to get updated to the most recent version. Then there's the issue of "Server legacy", where development of server (& the existing chunks) have to complete new features, blocks & biomes with the new versions... which will eventually run into 3 considered solutions: A) The "Scorched Earth" plan -- Throw out the old world completely & start anew. This can make players justifiably upset as they lose all their work in the process, but it creates a new world with untapped resources that can potentially level the playing field for the current player-base (with the typical "Free for All" land grab). B) The "Uncharted Territory" plan -- You leave the explored (& developed) world alone & let the server generate the new "unexplored" chunks with newer version. This is the easiest option as it requires no work to implement. The issues with this option is that it will create strange "world seams" as biomes can greatly & suddenly change with modifications to the world generation engine. There's also issue of world size, as if players have traveled a great distance from spawn or the world's "Origin" point (like 5 km or MORE away)... it's going to require players to travel further (thus, increasing the size of the server). C) The "Trimmed World" -- This is the most labor-intensive (& time critical) for the server owner, but it can provide a reasonable compromise. The server owner / admin takes a copy of the world & analyze each chunk to determine if it should be deleted (& reform to the new world generator) OR retained. This would allow players to retain most of their creations while getting the new content in the undeveloped areas. The only casualty would likely be any transportation lines (roads, railways, ect.) as the majority of those chunks are undeveloped. While it's ultimately the owner's decision on IF or WHEN they choose to upgrade their server based on what they want to do with their world, I hope this sheds some light on the subject.
They stopped playing and forgot about it.