Spoiler warning: it’s because of multiculturalism.
But actually, invocational systems of all types (verbal, inscriptional, and gesticular) are inherently dependent on language. Language shapes how we see the world, how we’re able to interact with the world. I’m sure people who have studied a second language (myself included) have stumbled upon words that are difficult, if not impossible, to translate into English. Or perhaps English words that are hard to translate into other languages (like defenestration. Seriously, who’s the prankster who made THAT word?) But this gives each language a unique beauty, like a handcrafted stainglass window.
Invocational systems smash those windows. All of them. And then erects a single window (that’s usually not even that good) in their place.
But enough with the metaphors. Invocational systems generally put aside particular phrases or words to have a magical effect. Usually, these particular phrases and words are in an “arcane language” not native to the caster. The problem here is that this elevates the characteristics of this “arcane language” above all others that may exist. After all, wouldn’t it be better to speak in a language that has magical effects than a language that doesn’t?
After all, wouldn’t it be to everyone’s advantage to be able to understand, read, and write in the arcane language, if only so they know what kind of curses or spells are being thrown around them? “Oh, hey, he said ‘fire.’ I should get ready for that.”
Systems that are primarily inscriptional or gesticular are even worse. Verbal systems restrict magic to those with the intelligence to grasp language and those lucky enough to either be born in the right time or place or be wealthy enough to learn the language (meaning animals that can actively do magic should probably be nonexistent); inscriptional and gesticular systems restrict magic even further, cutting out anyone who’s unable to move their body in the required way to cast.
Consider the Harry Potter universe. How do left-handed wizards cast spells? Is the “swish and flick” mirrored? Is it the same as it would be right-handed? In the Avatar universe, what happens when a bender is born without one or more limbs? What if a bender loses a limb? How does that loss affect their ability to bend? Do they have to learn everything all over?
But I digress. Some examples of books that do invocation well:
Eragon has the prototypical magic system I’ve described: an ancient race sacrificed themselves to make their language magic. However, the book is very aware of the issues with this I have laid out. One subplot involves a mistake the main character makes when casting magic: he uses the wrong grammatical case for a word, and so screws up the spell’s effect. When the magic system is based on a language that’s no one’s first language, this kind of thing should happen, and happen a lot.
Brandon Sanderson’s Warbreaker requires that all commands or spells be spoken clearly in the caster’s native language. This is largely justified in that 1) the commands are given to what are in effect pieces of the caster’s soul and 2) the commands are usually more than just one word, and are very specific and imperative.
On the flip side of that coin, Patricia C. Wrede’s Mairelon the Magician uses the complete opposite of this. All spells are single words that must NOT be from the caster’s native language. It doesn’t matter what language is used: all that matters is that the word is actively connected to the effect of the spell, as the prelearned connotations of the corresponding native language word fuzz the effects of the spell, preventing effect precision.
This isn’t to say invocational systems aren’t bad. But their connotations and subtext must be accounted, and I, personally, prefer systems that aren’t quite as culturally restrictive.