Any in-universe or even headcanon reasons why an entire race like Tsaesci, abundant at their peak as they were, could've completely fallen through the cracks of history to the point where nobody knows what they looked like anymore?

Now, I know that the meta reason is literally just "Todd wants a mysterious race to stay mysterious", but what about in-universe reasons Tsaesci (and Kamal) in Morrowind just stopped existing abruptly and nobody knows what they even looked like?