Instability.
They're volatile as hell and you never know what's gonna happen. Not to mention you don't know when the crypto bubble is gonna burst.
aside from they're not stable coins, most of the have no working products or projects.
That's why most altcoins aren't pumping because they have no other reason why their coin have the right to pump.
some altcoins have working products and projects and that made them listed as one of the top altcoins in coinmarketcap.com and those top altcoins are worth for more investments because they have a good crypto projects that somehow useful for some users and investors.