That's because Nubits pay anonymous people to run trading bots to provide fake volume and liquidity, also nubits require external exchanges to even work.
There's no faking of volume, bots trading to each other would lose money. NBT has high volume because it's very useful when other cryptos (Bitcoin) are being volatile.
BitUSD does not need trading bots, and also it trades on a decentralized internal exchange, in addition to external exchanges.
Exactly why it sees no volume. No one outside of the Bitshares world cares about it. " in addition to external exchanges" Oh you mean Bter? Cause that's it, and it has $18 in volume.
To those wondering how reserves work in Nu Jordan recently posted this.
https://discuss.nubits.com/t/regarding-reserves-and-fractional-reserve/1126