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.
I also often find coins that I think are suspicious because, the prices are not so good and the ones who use them are a bit so I am very reluctant to buy these coins. It is better for me to put coins that are more valuable in my competition so that I can reduce my losses because of coins like that it is clearly detrimental.