Gabe Newell speaks about it in the documentary. Here's an article about it: http...

Gabe Newell speaks about it in the documentary. Here's an article about it: https://www.theverge.com/2024/11/16/24298091/gabe-newell-hal...

Basically, they wanted more than just simply finishing the story, they wanted to make some cool new features, similarly to what HL2 was compared to HL1.