One is of course the network effect. I can't pay with (whatever)coin unless other people use it too. So unless I'm just really excited about crypto and want to pay with it, it's not very useful to me until more people start using it. And considering most of the crypt-hype has worn off, there's less and less people that are just dying to use it.
And the other is that people (and businesses) are not going to try things over and over until someone gets it right. Bitcoin is terrible as a currency. I tried paying a friend of mine for a sandwich once, just for fun, and it cost me $10 to send him $10, plus it took almost an hour. Of course it's less bad now, but it's still not great. Valve started accepting bitcion and later stopped. Lots of retailers did too. No one is going to jump on Bitcoin Cash or Monero or whatever, even if they're better, because they were burned pretty hard with Bitcoin.
Someone is going to have to genuinely nail it with a currency that is instant, costs almost nothing per transaction, is very easy to use, and they're going to have to make sure everyone knows it. And even then I think it's going to be uphill.