Network effects. If you're a darkweb seller you know most people want to send BTC they purchased straight from a KYC'd exchange, this is preferable as any extra friction is bad for business. You would lose 70% of your customers if you only accepted monero, which has been delisted from any exchange with a fiat gateway. You can always switch the funds into Monero later on anyway to save your own privacy. Your customers being pwned for using non-privacy coins doesn't matter, so long as they only lead back to your darkweb nickname.