1) 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.
> There are some pain points in the older version that 10 years of experience can fix in a newer format.
What were the major pain points? Compression ratio and speed seem like two of them. (Thanks for answering the elementary questions.)
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.)