Simply because they have no value, and you don't sell it anywhere. Some may sell but prices are too small for us to trade. So I think it's a reluctance to keep those altcoins in my wallet.
Through analysis of the situation, I have come up with the idea and question that, is there a chance that we could somehow revive the coin that is already lost and forgotten especially on coin market cap? If that so, what step should we need to implement in order to get back things up and make it good again?