One reason is that Premium is a Revenue Stream for this platform. Money has to come from outside to flow into the ecosystem. If they allowed $LEO to be used, a lot of people will use their LEO earnings instead of bringing more money into the ecosystem.
Currently, since it needs HBD, you'll have to manually convert your LEO earnings to HBD first, most people won't bother and just use their HBD earnings from HIVE Rewards, and that's what the team wants.
...If it'll make you feel any better, soon (if everything goes smoothly,) you'll be able to pay using $BTC and other Crypto coins instead of just HBD.
HBD it's new money in to the ecosystem? The Main reason for a Long therm rise of the prices for a token it's use cases, people cuold use leodex to swap usdt/usdc > $LEO and that is new money, and again use cases it's the main reason to not sell a token when he pumps