Because the dev of each token are very inovative they make way to get investor like bounty(signature campaign , social media campaign ,etc.) .. Some new token release or ico have also sale in there token  thats why many investor come  to invest to try there luck if it can make profit in that program and after they make a profit in that token they will leave it (I hate that because they ruin the crypto world after you've make some profit you jump in other token )