Elaysians come from a low-gravity planet, where they were able to ‘fly’. However, their physiology was designed for low gravity, and in higher-gravity environments, they require special equipment to adapt and work. As a result, few Elaysians leave their planet. Ensign Melora Pazlar was the first Elaysian to enter Starfleet. Doctor Julian Bashir had developed a medical treatment that involves elevating neural output to the brain’s motor cortex via neural analeptic transmitters, which would allow them to move as a normal humanoid in Earth gravity, but as yet no Elaysian has undergone this irreversible procedure.
You may have additional Elaysians join Starfleet after Ensign Pazlar, and it would be up to them whether they want to undergo the experimental procedure.