There are a couple of reasons.
Firstly, it got a buff in Gen 8.
Generation VIII
If the Pokémon sent out to replace the user already has full health and no status condition, Healing Wish's effect will not activate until a Pokémon is placed in that position.
Source
This means it's really helpful, since you don't have to worry about in coming in a Full HP Pokemon and now it's saved for the team. Secondly, Lopunny is much more of a support. It relies on Toxic Orb and Switcheroo it to the opponent. Since Toxic will already wear it down, so Healing Wish is pretty much executed before it's about to faint from Toxic.
Lopunny is much faster than how much it hits harder. It has a Spe of 240 in Gen 8 Randbats while its attack is pretty medium, at only 188. That's why Lopunny acts more like a Healing Wish Pokemon and something just to sack while spreading Toxic status to opponent's team.
Hope this helps.
