The lycanthrope has been a standard monster since the game was invented.
From OD&D Monster Manual(1974, Monsters and Treasure) (Lycanthrope):
Only silver weapons or magical weapons/attacks affect Lycanthropes.
From 1e Monster Manual (1977) (Lycanthrope)
All are hit only by silver or +1 or better magic weapons
From 2e monstrous manual (1989)
Hit only by silver or +1 or better magical weapons
3.5e SRD (ref is 3.5e. ~ 2003ish)
"possesses damage reduction that is overcome only by silvered weapons
(skp to) Damage Reduction: A creature with this special quality ignores damage
from most weapons and natural attacks.
In the case of a werewolf, the damage reduction is listed as "10/silver" meaning that some damage could be done with un-silvered weapons, but the blow is ineffective if it does not cross the threshold. This is similar to but different from "resistance" as played in 5e. At low levels this can render a lot of blows ineffective (failing to do at least 10 damage) unless using silver or magic.
(I do not have any 4e references available, 2008).
Damage Immunities: bludgeoning, piercing, and slashing from nonmagical attacks not made with silvered weapons.(DM Basic Rules, p. 49)
Some design features of D&D 5e
This edition tried to fold in some of each previous editions' features, which is related to some of the design goals for D&D 5e. Uniting the editions was one of the design goals.
- One of the things that returned in 5e, in our experience, is an
increase in lethality, particularly at low levels. (Anecdote: Our
initial encounter with wererats in 5e at 3rd level forced us to use our
melee fighters to hold them off while the casters tried to take them
down. Plenty of our hits did no damage).
An important design goal was simplicity of play. The constant reference to "plusses and minuses" (like DR) does not aid and abet simplicity. Immunity and resistance is simpler than an extended list for damage resistance. So too is the use of advantage or disadvantage for combat. While the advantage piece isn't part of this particular detail, it fits with the "simplify play, speed up combat" design mode. Applying this line of thinking, a return to the traditional lycanthrope is less complicated and achieves a bit of that "some stuff from each edition" theme.
Could they have used resistance rather than immunity? Yes, though doing that might not go as far toward the "unify the editions" theme. The immunity makes the lycanthrope a much scarier monster for low level parties.
Is the monster manageable by a typical 5e party?
As @LegendaryDude pointed out, silvered weapons and magic damage are easy to come by and affordable for a party of average level 3, which happens to coincide with the CR for a werewolf. Not only that, but plenty of classes in a party (the game was designed with parties in mind) will have access to some kind of magic damage, via a cantrip or a low level spell. Silvered weapons cost a bit more than regular weapons, but are not overly expensive.
Or, they could try to push it off of a cliff. (But there's never a cliff nearby when you need one, right?)
A monster is immune to damage from nonmagical bludgeoning weapons.
Does it still take damage from falling? Yes, that monster is still
going to feel the hurt of a fall, because a fall is not a weapon.
Is the 20th level barbarian at risk?
Your 20th level barbarian, in the "experience" he gains by going up all of those levels, will learn that he needs a silver (or magical) weapon to damage a werewolf. A dagger is all he needs, at that level. If the barbarian can't afford a silver dagger -- maybe in a boot sheath -- by the time he's 20th level, he needs to look at his gambling and drinking habits, to say the least. Maybe he needs to invest in a silvered axe.
Basic Rules, 5e, p. 47
Some monsters that have immunity or resistance to
nonmagical weapons are susceptible to silver weapons, so cautious
adventurers invest extra coin to plate their weapons with silver. You
can silver a single weapon or ten pieces of ammunition for 100 gp.
This cost represents not only the price of the silver, but the time
and expertise needed to add silver to the weapon without making it
The other point in 5e design is bounded accuracy, which means (among other things) that high level characters are still in some danger. While this is a poor example -- 20th level barbarian versus a werewolf -- a whole pack of werewolves is another matter. They might still take him down.
Werewolves are scary creatures. That's a good thing. (Warren Zevon would approve).
As an aside: in 1e's Unearthed Arcana (1985; p. 20), at 4th level
[Barbarians] May strike creatures hit only by +1 weapons
There are some precedents set to add such details and features to overcome lycanthropes' invulnerability to mundane damage. That said, another design goal for 5e was simplification, not extra fiddly bits.
If it's any consolation, that feature for the Barbarian is one that I wish they had resurrected from the TSR graveyard.