Supported by and easily transferred across all major blockchains. Practical, everyday use cases. Lends itself to enabling a fiat wallet as well. Yes, USDT overshadows USDC as a percentage of global stablecoin 'volume', but USDT has its issues. Long term, it seems USDC, or a new entrant, emerges, but so long as Proton can avoid issues making USDT available (in addition to USDC, and perhaps one or two others), USDC + USDT + fiat seems useful for the broadest audience.