I think the distribution of saving throws is something that has been implemented rather carelessly overall in 5e. Take a look at "conventional wisdom" many players use to gauge which saving throws are worth having good numbers in- Strength saves generally prevent forced movement or being knocked prone, and occasionally might prevent/mitigate damage. Dexterity saves almost always simply prevent/mitigate damage. Constitution saves are important for spellcasters to maintain concentration, and can prevent/mitigate damage (often poison) or prevent some nasty status debuffs like poison/paralysis/petrification/polymorph.
Intelligence saves are rare, but are not something you want to fail when they come up, as they are often in the "nerfed beyond all playability/unable to act" category. Wisdom are much more common, especially for player spells. Charisma saves are basically the same as Int saves, but more common (but still rarer than Wis saves).
This disparity continues when it comes to what spells players use. They might avoid using spells with Str or Con saves- one, because the benefit of a failed Str save is often not all that great, and two, because most players think that few monsters have bad Con saves*. Also worth considering is that immunities or resistance to Con save effects are common- many Undead sneer at Necrotic damage (but not all!), and Poison resistance and immunity seem common enough to worry about. Immunity to Poisoned, Stunned, and other Con-adjacent conditions are not uncommon either. Many of the good damage dealing spells require Dex saves (but there's a lot of debate on how good damage dealing spells are in general that I won't get into, beyond noting that the debate is).
As noted, there's a lot of Wisdom-saving throw spells, and fewer Int or Cha save spells. Debilitating conditions from Wisdom-saving throws can suffer from random immunities as well (Charmed, Paralyzed, Stunned, etc.), and you can't always tell if something is going to resist such an effect by eyeballing your opponent.
What I've noticed is that my players get frustrated when they use spells that require Wisdom saves because even if the monster doesn't have a good Wisdom save, the chances of it being randomly immune to whatever the spell does is always present.
What you are fighting matters, of course, but I started to notice that less and less Wisdom save spells were being used because of this, especially after a grueling adventure where they had to deal with a lot of Fey and "sylvan" creatures**.
The point is, if nothing is particularly vulnerable to a particular effect, then players may grow disheartened and stop even trying. The idea that a Rogue-adjacent critter like an Assassin might have terrible Wisdom saves (not unlike the Rogue class) is a good one from a design perspective because it's something you can guess might not have a good Wisdom save or any weird immunity to said spell- but runs right into the fact that there are so many Wisdom save spells to begin with!
I don't think any particular thought was put into making sure there was a more even distribution of save types for spells- 5e is more concerned with making all the classic spells available, than making sure that players have a wide selection of spells in every category that just fracking work. Synaptic Static might be the first Int save spell a player ever uses, at 5th-level, and they'll likely keep using it once they start, because while it's damage is lackluster, it's got a nasty debuff and it's a save few creatures are going to make. What spells have Charisma saves in the first place often feel picked at random (we don't know what the save should be, so we'll just say Charisma so it's not a completely useless save seems to be the general logic, lol).
No effort was made to give players a lot of Str-save spells, and like Con-save spells, they might quickly find it's not worth casting them anyways. Add on top of all of this the swinginess of the d20 due to bounded accuracy- something with a "good save" (high stat+proficiency) probably only has a 50% chance to save in the first place (barring nonsense like magic resistance), and even someone with a "bad save" can get lucky (until they can't, because the DC is too high, lol).
Because spells are limited use and many have zero effect on a successful save, monsters (in general, not specifically) need to be vulnerable to them or it's just a waste of resources to use them. The flipside of this is the existence of spells that just "win the combat" if you fail a saving throw. As a DM myself, I understand the razor's edge the game is balanced on in this respect- you have to make spellcasters feel useful as a character choice or nobody is going to want to play one, but if your monsters consistently fold in half when presented with an offensive spell, the whole affair feels like a waste of your personal time.
Contrast and compare AD&D, where the vast majority of spells are "save neg." and monsters had very good saving throws that only got better over time, and that's before you even discuss old school magic resistance and random immunities some game designer felt a monster "should have", but many adventures hinge on having someone able to cast a specific spell (or spells)! 5e is better than this, but still an imperfect game***.
*I've noticed that Con proficiency isn't all that common among monsters, despite what players (even myself, when I'm playing) might believe. But it's pretty rare for their not to be some bonus. The resistance/immunity is really the bigger concern when using such spells.
**Such creatures tend to have pretty good Charisma saves, and magic resistance is not uncommon.
***I'm not saying the game has to be perfect. I don't know what that would even look like! Obviously, enemies that snicker at spellcasters need to exist, just as enemies that snicker at martials should also exist, but you need to be careful with this, because it forces players to be careful with what classes they elect to play- something not every group is really going to want to do. Even among people who have played for decades, I personally often run into party optimization beginning and ending with ("who gets stuck playing the healer"- with said task being so odious that more and more groups I'm in have NPC healers and/or force someone to play two characters)!
Not long ago, I heard someone say "you know, I've never played an Enchanter Wizard, I think I want to try it" and have everyone in the room look at them and hiss dire warnings against such- myself among them! Whether or not their spells would even accomplish anything against a random distribution of foes is suspect to begin with, beyond even getting into the player vs. DM minigame of figuring out what you can expect a charmed/commanded/suggested NPC to do) and then, if the universe aligns to make their spell actually work, it's usually to the tune of "well, so much for challenging the players this time!".
In a nutshell, that's the problem I see with how the game really handles spells. And with modern game design being so dead set on making more options for spellcasting PC's than not, it just gets exacerbated.
