Because killing the Fire Giant does not burn the Erdtree. They're seperate actions, and the latter requires doing the former.

I'm guessing they went do other stuff (like the consecrated snowfield) and then got beaten so badly by Malenia and/or real-life obligations, they just never continued

The first thing I did after beating the giant was fall to my death jumping down the cauldron. I imagine some never came back and got bored. 

Fire giant was a hurdle and they decided that was enough? Or they didn't know how to progress lol

When you get ready to burn the tree, there's a speech from Melina implying it's a point of no return (the real point of no return is later, defeating Maliketh). I imagine players, including myself on first playthrough, believe her and go away to do other stuff.

It does stop a few quests I believe,