Paladin had a 22 AC, protection from evil (giving disadvantage to all undead attacks), and heavy armor mastery (-3 damage per attack). He was standing in the doorway, so could not get surrounded on all sides (and we don't use alternate flanking rules). And of course can heal himself. Note that with a wight's +4 to attack, that is only a ~2% chance to hit with each attack, and on average it does 3 points of damage.
As the paladin was blocking the doorway the other wights could not charge in as you say. So they chose to focus fire, as you noted it felt the best way to try and drop a key player quickly. But the paladin was too tough.
Now could I have played it more intelligently of course, the DM can always "win" if he wants. I'm just going back to my original premise, 5e can be deadly, but you have to work at it. I think most DMs would say "hmm....20 wights, and a wraith? Damn, that sounds pretty darn deadly". But it wasn't even a challenge in this example.