Unity UI (uGUI) was the initial reason I was hired at Unity. Being part of the team that had built NGUI — the most popular Asset Store package at the time — we were the best choice to deliver on the promise of a more efficient runtime UI framework.
After a few weeks reviewing the state of the unreleased replacement, it was decided we needed to start from scratch. The next 18 months were spent designing and building all aspects from the ground up: rendering, input, layout, and editor workflows. Extra care was taken to ensure the system was flexible and extensible, since the team knew they could not support every use case on their own.
After release, uGUI quickly became the UI framework used across the Unity ecosystem and is still actively used over a decade later.