Singlehandedly probably one of the best things ever from that stream, and that is so much coming from a superb stream.
"Why did you die?" is so reminiscent of a question a child would ask if their parent had died, whether from suicide or other means (in this case, suicide).
It is a question of anger and frustration and pure not understanding. it's the same thing a kid would ask because they don't understand how a person could hurt so much to do that, or how they could leave them.
Fundy was angry and mad because Wilbur left him. he didn't know why he left him, he didn't know or understand why they couldn't do something else, anything else. He didn't know that Wilbur was hurting, he didn't know, and in those moments he didn't care. it's the question from a hurt child.
In that moment, that's what he was. a kid suddenly very alone in the world, knowing that the only person he had just left him, and everyone after would follow suit.
"I was very sad." Is such an... honest answer. It suffices. He doesn't have to say 'i was paranoid, i was depressed, i was tired, i was hopeless'. He said what was true, it weighs just as much as everything else simply because it's a fact not many people knew or picked up on.
Wilbur was honest with his intentions in Pogtopia, but not his feelings. When he was revived, he also made such a point to pretend he hated something just to keep up this sort of image that he never cared and that it hurt him how much he cared. But... that's the point. He was sad, and he was hurt.
Not only is this a punch in the face hearing Wilbur say this, but it's the only real time we've heard him say much of anything about what he felt during Pogtopia (similarly to Tommy and his silence on exile. in my opinion, this felt very similar to 'it was a wiggly time in my life).
But it also fits in line with the previous comparison. It's such a simple explanation that someone may use to explain to someone, like Fundy, who didn't understand why someone would leave them in that way. It's similar to how a family member may explain to a child why their parent is no longer with them.
It's not too detailed or grim, but it's not lying or sugar coating it. It's simple, and true, and an answer that Fundy, all too clearly, understood.
It's even more impactful, gut-wrenching, with the follow up question,