It's because all of them mooned in December '17 to Jan '18. New moon boys join the ride and blindly buy anything because it seems any coins can make them rich quickly. After that, they crashed dramatically altogether and a lot of people leave the market because they think this is another failed experiment. The graph is quite funny though.
Unfortunately, I was one of the many who assumed at that time that things will continue going up and will never be coming down and that there will never be bubble at all. We forgot that this is still dealing with real marketplace and with real people. We forgot to learn the lessons of the history of investing and thought that we could ber supermen inj many aspects. And then we were surprised with great disappointments later. One factor here is that at that time many newbies were really got involved with the cryptocurrency market without the very basics of investing. I am wishing that this year all of these can be rectified so that comes 2020 the cryptocurrency market can be made up of people who know what they are doing.