Actually, HBD is listed on some exchanges, just not many. Many people actually run bots trading HBD on those exchanges, because it is possible to profit from the "stable coin" value and temporary price variations on the exchanges (one of the most well-known examples of this is the HBD stabilizer).

But the reason HBD isn't listed on many exchanges isn't a legal issue, nor something related to Luna, etc. How do I know it has nothing to do with LUNA, etc? Because the same situation was true long before LUNA came on the scene (HBD has been around for many years).

The primary reasons it hasn't been listed on many exchanges is because 1) as a fairly "stable" stablecoin, it isn't particularly speculative, so it doesn't generate a lot of trading from speculators and 2) it doesn't generate a lot of volume that a stablecoin may get by acting as a stable source of value as traders temporarily seek safety between speculative buys because the existing supply isn't large and its not been used as a common-coin across markets on an exchange (think of the way USDT has had a trading pair with a lot of speculative coins on several exchanges).

So even if HBD gets listed on an exchange, its usually just listed either USD or a stable coin and maybe a BTC-HBD pairing. But for a stable coin to generate volume (= profit for an exchange), it has to have pairings across all the speculative coins.