Because Liberty had take so much place before GnK. And after some testings, it was obvious that the Tradition tree was the perfect fit for peaceful and science focused games, though the original idea was to build the most productive cities. So it was time for Tradition to regain some noble stories
Liberty was always be the #1 tree for any kind of victories, even when
Landed Elite was op before they changed it(some months after release of vanilla). Liberty's biggest advantages was the free settler from your 2nd policy and free gp who doesnt delay your 2nd gp. It was incredibly strong.
Liberty is stronger for dom. victories because you can keep your early gold to rush buy and upgrade units instead of settlers/workers. Just make sure to keep the train rolling through eras.