The top 5 reasons paladins take it (in no particular order):
1. To do what they want while avoiding silly sheninigans like dropping weapons during combat
2. For a big early bonus to concentration saves
3. Because their DM rules that having the feat allows you to cast whatever spells you want while holding weapons or a weapon and shield in both hands
4. There's a spell they really want to be able to cast as a reaction, possibly booming blade picked up through multiclassing
5. They don't like the visuals of sheathing their weapon in combat to cast a spell
My guess is that some combination of those reasons are why most every player choose the feat.
1. I understand for players who don't like role play in there table top strategy game. They exist, but enougth for #1?
2. Sure lower level characters are more common. Actually a pretty valid point. I tend to think this is the best reason though resilience would also apply to other con saves, knowing that I would wander why it didn't make the top 3 if warcaster did for this reason?
3. Two handed weapons don't require 2 hands just to carry but to wield so all the paladins not using a shield don't need a feat (polearms masters being the 3rd most picked feat and great weapon masters) add to that not having many spells that you would need to cast more than once a battle with a somatic but not a material component. The only good one that has been pointed out was Dispel magic. Cure wounds can be ignored with lay on hands for the most part and everything else is ether better served out of battle or would generally last the whole fight. Largely your talking about 1 maybe 2 casts in a generally melee and smite fight.
4. Multi classing ... that is a consideration.
5. Hu? Its schematic they do it in movies and video game trailers all the time just because it makes for cool, "Now he is really serious!" moment.. but no accounting for taste.
That said, you do realize 1,3, and 4 are basically table "because". I would be more surprised if they were enough to push any feat to the #1 spot than anything.
So I can see some of #2 but not on its own and a lot of #4 pushing it higher than I might have thought. A booming blade sorcerer smite might not be optimal but for people who like theory crafting it seems like something people would want to try. I can also see that shield, hellish rebuke, counter spell, and Absorb Elements being pulled form multi-classing and making holding the weapon actually matter since actions but not reactions let you pick it back up. As well as adding spell options to make opportunity casting more appealing like booming blade as you mentioned.
I was defiantly thinking of paladins as just paladins but considering all the 2 lvl multi-classes dips in paladin for smite I have seen ... that makes a certain since. They don't show for example top 3 feats for paladins with 8 levels of paladins vs paladins with the first 2 in paladin and the next 8 in sorcerer,bard, or warlock. If they did it might confirm that as the reason its higher than I would expect.