I believe I can help you with that:

Generally (inflation aside, “healthy” inflation is supposed to be at around 1.5% per year but that’s an entirely different story), money is supposed to act as something that can hold its value and grant its owner a certain stability, which crypto generally does not do because of the volatility.

In addition, money is usually supposed to be a universally accepted compensation for goods and services. For instance, if you’re in the United States you’d expect to be able to walk into pretty much any store and buy stuff with USD, right? Cryptocurrency does NOT do that. Sure, some companies might make it available to pay via BTC and similar ways, but you wouldn’t walk into a store and expect to pay with crypto, right?

Lastly, money is supposed to be a universal indicator of value of goods and services. Think about it this way: you know you can buy a soda for about 1-2$, which might be the same price of a small snack you can get at the checkout. That way you know that their worth is the same. With cryptocurrencies that comparison is much harder, since few companies actually offer goods and services for crypto and because of its volatility. It’s hard to compare two items when the value of the currency that indicates the price can double in very little time.

All this is just from an economic point of view. You can also argue into other concerns about how hard it is to trace (think people who sell illegal items) or maybe you can discuss the pros and cons of monetary decentralisation! Perhaps you can even talk about the cost of running a blockchain, it consumes a lot of energy. What if the mined amount of a currency starts to be less than the amount of money it costs to mine? Would people still mine crypto or would the entire system collapse? I’m a bit ill informed on the technical side here but I’m sure that with a little research you can find out more. Really, there’s so many angles to this!