alt Hacker News1) It's not. Maplibre is a JS library for displaying map data. OpenStreetMap is a collection of map data that is published in various formats. Different levels of the stack.
2) It's an optimization/advancement. There are some pain points in the older version that 10 years of experience can fix in a newer format.
3) Attention, funding. Technically, they're at the leading edge of open source.
Additionally to point 2, the older format was created by a company (Mapbox) that used to be open source-friendly but has recently made a larger pushback against open source and open standards, changing the licenses of much of their formerly open source work. (The Maplibre JS library itself is a fork of that company's previous open source work from its last open source drop to keep the work open source.)