Why this forum deletes full threads ?

perhaps because you raised some sensitive issues related to someone who‘s products are heavily advertised on Audiogon

Perhaps that particular ‘Someone”had a hand in that deletion 😜

Because it’s their forum/place of (unregulated) business and they can delete whatever they want. Or OPs can delete their own threads anytime normally anytime they want. Best to ask the only people who could give you an answer if needed ie the moderators, if desired, but I would expect they are under no obligation to explain such things to anyone if they choose not to. It’s a free forum that serves a business purpose so consider using it a privilege not a right.