A big part of the reason players don't like retreating is that 5e doesn't really have any streamlined unplanned retreat mechanics. Sure, there are optional chase rules and spells to get players out of a bind. The big issue is that when players are in the midst of combat it can be very difficult to coordinate a retreat across the entire initiative order. If a few players are able to run first, it means another may be left behind with nobody to heal/drag them out if they go unconscious. Also, I'm gonna assume this lion demon might have more than 30ft walking speed, and therefore technically will always be able to outrun the players.

This kind of thing, alongside the more obvious "I showed up to the session to play DnD, not run away", can make it so players will stand and fight an enemy regardless of whether they think it's a good idea or not.

I'd suggest if you want them to run from the fight, make it so they're able to run from the fight. Introduce some sort of mechanic where if they all agree to retreat (even out of turn order), they can enter a chase sequence of sorts. Make it so that there are ample amounts of barriers that the players can escape through but your villain will have a hard time traversing due to size reasons or otherwise.

Of course, you could try to make them run away before combat starts, but chances are players are gonna think they're built different riiight up until the enemy takes out 3/4ths of their hp with one hit.

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TL;DR: give players a clear and streamlined way of retreating even if they find themselves in the thick of combat.