What else should I be? All apologies...
This is a bit different from my usual answers. Typically I attempt to determine what the rules text actually says, in the most literal reading possible. As the querent notes, however, Compelled Duel is different1 in that it "does not actually say what it does". Thus this answer is an exercise in apologetics - what is Compelled duel trying to do (RAI) and how can one read its text more generously so as to permit it to actually do that (RAW)?
Hopefully we can all agree on what compelled duel is supposed to be doing. It is a paladin spell - it is supposed to be about the party's paladin challenging the opposing side's leader or champion. Drawing them off to face the paladin solo, keeping them occupied both to protect the squishier members of the paladin's party from their powerful attacks and so that all the other members of the paladin's party can concentrate on eliminating the minions. If we agree on the intent, how can we read the spell text in a way that allows the spell to do this?
You attempt to compel a creature into a duel. One creature that you can see within range must make a Wisdom saving throw. On a failed save, the creature is drawn to you, compelled by your divine demand.
The querent is concerned about the phrase "drawn to you", since in his reading this is either a statement of mechanics that the spell does not actually employ, or mere flavor that doesn't do anything but guide later interpretation. Let me suggest a third alternative; "drawn to you" is not just flavor, but it is not compelling movement, either. Rather, "drawn to you" is used in the sense of affecting the target's feelings and emotions but in a non-mechanical way. Consider it like the phrase in the spell charm person, "The charmed creature regards you as a friendly acquaintance."2 This phrase is not just flavor, but neither is it mechanics. Rather, it is a guide for the DM in determining how to role-play the behavior of the target; it is a suggestion for how to interpret their spell-altered perceptions. Similarly, "drawn to you" does not have a mechanical effect, but it is telling the DM that the target feels a strong incentive to attack the paladin. The spell is influencing2 - but not controlling - their behavior.
I DM a (vengeance) paladin who doesn't make use of the spell. But I am also a player in a game where another player has a Crown paladin who gets compelled duel as an Oath spell and makes frequent use of it. What my DM does is simply have the spell 'work' on any brute creatures that fail their save. They are drawn to attack the paladin - so they do, having no reason not to. These are the same kind of foes that are otherwise indiscriminate in their attacks and which would normally be attacking the nearest opponent with little regard for tactics. More intelligent foes under the spell would still feel drawn to attack the paladin, but can recognize this urge for what it is and still make decisions in a logical manner - 'ok, I can attack the paladin normally, or another target at disadvantage, what makes sense to do now'?
Turning to the actual mechanics of the spell:
For the duration, it has disadvantage on attack rolls against creatures other than you, and must make a Wisdom saving throw each time it attempts to move to a space that is more than 30 feet away from you; if it succeeds on this saving throw, this spell doesn’t restrict the target’s movement for that turn.
It starts with the simple disadvantage to attack anyone but the paladin, which is clear, and concludes with the more opaque effect on movement. The target can freely approach the paladin from any distance, and they can likewise circle the paladin, moving but maintaining their distance. If they try to get further away than 30 feet, though, then the spell imposes a mechanical effect; they must make a Wisdom save, with success indicating that their move is unrestricted. As noted by the querent, "the spell never tells us what happens on a failed save", at least not explicitly. But we can tease that out. First, because the spell tells us that this effect is triggered by an "attempt" to move further away; if it is an attempt, then it has to be possible to fail. Thus failing the save here means that the target cannot in fact move away. If success on the save means that the target's move is unrestricted, then it follows that failure on the save means that it is restricted, and restricted must mean that cannot move farther from the paladin.
Admittedly the phrase "each time it attempts to move" is odd, since as HellSaint points out, the target could simply keep attempting to move away until it succeeded on the save, since there is no movement cost to the attempt. However, note that a successful save here means the movement is unrestricted "that turn" and that the spell lasts up to a minute. Thus it seems reasonable to conclude that a failed attempt to move away precludes another attempt for the remainder of "that turn". To reconstruct what the spell is trying to say, then, it is something like:
For the duration, it has disadvantage on attack rolls against creatures other than you. Also, the first time on each turn that it attempts to move to a space that is more than 30 feet away from you it must make a Wisdom saving throw; if it succeeds on this saving throw, this spell doesn’t restrict the target’s movement for that turn, but if it fails, it is unable to move to that space or or any other space that is more than 30 feet away from you for the remainder of that turn.
This attempt to read the spell in a way that actually supports its intent concludes two things. First, "drawn to you" is to be understood as a non-mechanical influence on the target's motivation that requires DM adjudication for a role-playing outcome. Second, the mechanical effect on movement is such that a single save to move away may be made each turn, with failure restricting the target's movement options to only those that don't place it further away from the paladin.
1Starting, of course, with its name. Compelled duel does not compel anything, and what it does affect mechanically is movement, not selecting opponents.
2And note that compelled duel, like charm person, is a spell from the school of enchantment, which by definition seeks to "magically entrance and beguile other people and monsters" and which affects "the minds of others, influencing or controlling their behavior."