The fundamental flaw with all current crypto is that it's far too volatile to use as a currency. The only reasonable use for it at the moment is as a high risk commodity which is the vast overwhelming majority of what we see. Any so called "currency" that regularly sees price swings of multiple percentage points in a day isn't actually a currency and is unsuitable to be used as one.

Adding to this is the problem of transaction times. Actual payment systems typically have transaction times of less than a second, occasionally a second or two. Bitcoin in contrast can take multiple minutes, sometimes hours or even days to confirm a transaction. There's no way for Valve to accept and then immediately convert Crypto to USD. The process would inherently involve at least two transactions, one to transfer the crypto to Valves wallet, then a second to transfer from Valves wallet to the exchanges wallet, and only then could Valve attempt to sell that crypto. The financial uncertainty involved in all of that is entirely unacceptable for a business.

At this moment there is only one potentially viable way of approaching this and it's government regulation of some kind. Either government needs to regulate that payment processors get no say in the contents of customers business, or else they need to regulate the adoption of a neutral digital payment system. One possible example of what that could look like would be the GNU Taler system which might eventually become a payment system in Switzerland but isn't yet.

It was the support cost as far as I remember. Way too many people were too confused about how Bitcoin works.

Volatility was not the concern, at least for Valve. They've utilized a payment gateway that just swapped the BTC to USD right away. Which was still a single point of failure, but in case of bitcoin, the company switching a payment gateway does not affect the UX for the customer as much.