because all these altcoins that are being created non-stop are not good projects. they are mostly shitty projects (hence the name "shitcoin") with nothing to offer and the rest of them also struggle with many issues which prevents them from gaining any real adoption and staying 100% speculative tool for traders to play around with.
them I suggest you to invest in top 100 coin. all of them even have a nice project and better than bitcoin. bitcoin is actually a Ponzi scheme that the reason its increasing drastically over a year and its drop so much. people are afraid with this bubble. imagine how many people are loss opportunity and money when investing on this ico.