Because they are not really decentralised. Vast amounts of cryptocurrencies as they are designed right now are disproportionately held by the early adopters, who will inevitably engage in rent seeking in one way or another (I wish I was wrong on this one).

Decentralisation has to happen through increasing trust between people, while cryptocurrencies are designed to replace trust. Yeah, current institutions controlling our monies are hardly worth the trust, but honestly bitcoin doesn't even remove that trust (it just shifts it to some random self-proclaimed experts at Tether, twitter gurus or god knows what else the crypto people are up to nowadays).

I think everything you said is correct about Bitcoin, but is that true of cryptocurrency in general?

I think Bitcoin, which has its obvious flaws, is sort of the de facto flag-bearer of crypto, and more of a harbinger of things to come. I'm not yet convinced that Bitcoin will be the only cryptocurrency in widespread use, in the same way that the USD isn't the only fiat currency used in the world today.

I would like to see some sort of cryptocurrency to be successful, just to satisfy my inner nerd, but I'm afraid most of its problems might not be technical in nature.

Cryptocurrencies have potential to drastically change how recognition of value is created and distributed, so that's definitely an avenue to explore. Most of the big cryptos that are out there today don't really try to explore that, and currency is something external to value generation and recognition. I'd argue that even fiat currencies are actually closer to the value generation (as the money creation happens when banks give out loans, which at least are supposed to make world better). Purely from cryptocurrency perspective the only thing that it values is keeping the ledger alive. Exactly those self-sustaining by all means mechanisms are what leads to rot in many areas of life (academia, politics, corporations, you name it).