As mentioned, some versions of the game introduce breaking concepts that earlier players may not want to deal with (either because it breaks save compatibility, or they don't like the mechanic, etc).

Minecraft has this somewhat also, with some people sticking on various versions because of mods, or play style, or combat, etc.

For example, one huge change was going from a 2D map to a 3D one, another was how world generation was done.

See "Eras" here for the big ones: https://dwarffortresswiki.org/index.php/Release_information