That is mostly a technology issue. There is a maximum bitrate at which the transmission line can support for a given distance. To get past that, they have to start dragging fiberoptics to nodes they build closer to your house. Before you hit that speeds, protocol upgrades were giving you faster speed.

And I wouldn't say "very select regions." At least a quarter of the country live in areas with FTTN or FTTP networks deployed by the telecoms.