I was listening to it a few weeks ago, but vaguely there are auditing companies in the Netherlands that need to verify companies above a certain size whether they are handling their money properly. As I understand it includes tax accounting.
These auditing companies don't like cryptocurrencies. There are several of these that don't agree to audit Proton even because they are accepting Bitcoin, but none of the remaining would accept it if they were also accepting a second cryptocurrency.

Now that I think of it, it might have actually been the reason they don't accept Monero as a payment? In that case, the reason for Proton Wallet being bitcoin only has something to do with another wallet's developers having been jailed recently for handling multiple cryptocurrencies.

I recommend you to listen to it though, if you understand english speech. There were interesting topics (and Opt Out generally has interesting episodes).
This episode is 54 minutes, audio only. You can find it here: https://www.buzzsprout.com/1790481/15505787-proton-wallet-w-andy-yen.mp3.