There are only 2 kinds of cryptocurrencies a sane person would invest in.

1. Bitcoin
2. Transaction cloaking alts

There's huge real trade behind those.
Bitcoin market cap is covered by tens of thousands of businesses and small sellers. Both whitehat and blackhat.

ETH has market cap of $27,000,000,000 just because it attracts so many losers waiting for miracle .... the miracle had happened for few of them.
IRL total net worth of ETH holders is probably 1% of market capitalisation.
And ETH is a vehicle to funnel money from tons of suckers into hands of handful of IPO scammers.
And guess what the scammers do with their loot. Dump it.

Alts based purely on speculation have real value of not more than few % of their market capitalisation.
If I dump 10,000 Bitcoins, the value won't change much and the drop will last couple of hours till the buy/sell orders goes into "equilibrium".
If I dump $40,000,000 worth of LTC, the value will go down by 50+%. And the buy/sell orders won't meet "equilibrium" for weeks .... if ever.