I appreciate the OP's bewilderment, I do. A similar thing happened to me some years ago: I had posted a question which attracted a fair number of comments, the comments were nearly all on-topic and I thought really useful for anyone intrigued by my question. But the list of comments continued to grow until a moderator transferred all of them to chat.
When I look back at the chatroom created in 2015, there is the following public message:
This room has been automatically frozen for inactivity
The last message was posted 1669 days ago.
Instead, if I visit the OP's chat room I can see the system has deleted it again. And yet, I can read all of its contents. I have no idea why this is so, maybe because I have >10K in reputation?
So, I look back to the 2015 chat room, and find a valuable clue
Bingo! If numerous comments were posted and transferred to chat, the room will not be deleted, it is, instead “frozen”.
Further digging reveals
From the Chat FAQ: (emphasis mine)
Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. A room is considered worth retaining if it has more than 15 messages by at least 2 users.
Rooms not worth retaining which are inactive for 7 days will be deleted. Rooms worth retaining which are inactive for 14 days will be frozen. Frozen rooms do not allow any new messages to be sent, and are not shown in the default room list to prevent cluttering the rooms interface.
Quoted from userZizouz212's answer on Stack Exchange Meta (2015) Things may have changed since then...
However, I think the OP's recent edit replicating the very comments which he found most helpful, is rather distracting and a reader might be forgiven in thinking that they are part of the answer. Speaking personally, if the contents of the room are visible to everyone, I would leave them in the chat room or I would write an answer which also acknowledged the help of different users.