This is where I stumble with monsters. It would seem that conditions and damage type (and associated resistances, immunities, vulnerabilities) are key design aspects of wotc editions (brought over from magic the gathering,
as this article describes). Yet, bringing them into play, at least on the monster side, appears as unfun. That is, imposing a condition, or a monster that has immunity to certain damage types, renders characters "useless" at least for a round, and that is to be avoided. Yet, if you don't give monsters those types of abilities, they seem boring.
It would be a good way to design a game, but DnD doesn't go there for a specific reason:
Let's say you have a well-worked-out elemental rock-paper scissors system (a la Pokemon, but less detailed). Every monster has an optimized damage type to use against them: orc should be hit with acid, goblins with cold, gnolls with fire, etc. If you get good at descriptions (which boxed text can help with), you can make it a fun little puzzle to read the clues and figure out which to use. *
Then the wizard picks the right spell, the alchemist pulls out the right bottle, and the fighter selects the correct magic sword form his golf bag of magic swords, each doing a different type of damage.
And there's the problem: only spellcasters can natively engage with such a system, and most players would be reluctant to allow the fighter enough magic weapons to to have choices in damage type. Most people expect warrior-types to have one weapon they always use, which means that should almost always be useful.
Like I mentioned upthread, being shut down gets old very fast. One battle per tier of play where your weapons are useless is fine and fun and memorable, but if it's every level it's going to be frustrating, and if it's two-thirds of fights no one will stick with the Fighter class.
Since I also don't know how to have this cake while eating it, I agree with WotC's decision to not have damage type matter all that much.
* rolling a knowledge check to figure this out is a boring way to handle it, as is randomly guessing until you stumble into the right one. Trolls are annoying to run
anyways, imagine trying to run every monster like this!