Fiat means giving the payment processor significant leverage over the paid email service.
Fiat also means the email service couldn't be global, because Trusted-Third-Party-based payment processors can't operate globally owing to the global regulatory patchwork.
Contrast that with Bitcoin(Cash)-based international remittance, or Ethereum-based token sales, which are accessible to people in every country in the world.
Email is an open and global protocol, so cryptocurrency is a good fit for it.
They were a blockchain startup, so it'd have been embarrassing for them to base their service around the sort of money the average person actually uses. Also gave them a natural audience for targeted ICO spam.
Off the top of head, one reason is that there is no need to support multiple currencies for international users, or to worry about currency conversion.
Secondly it cuts out the need for middlemen (banks/payment processors).
Fiat also means the email service couldn't be global, because Trusted-Third-Party-based payment processors can't operate globally owing to the global regulatory patchwork.
Contrast that with Bitcoin(Cash)-based international remittance, or Ethereum-based token sales, which are accessible to people in every country in the world.
Email is an open and global protocol, so cryptocurrency is a good fit for it.