Why would a language created by humans lack both the /k/ and the /g/ consonants?

Question

In my story, there is a species of aquatic humans often called merfolk (Homo maritimus, so they are still humans, just not Homo sapiens). They do not look the typical descriptions of merfolk (I want to go on a semi-realistic way instead of a psychedelic-like creature as Princess Ariel from a famous Disney animated film): they are MUCH more massive than anatomically modern humans (they are as large as adult belugas), they have a seal-like blubber, they have webbed digits (both toes and fingers), they do not typically have visible hair, and they can drown (it just takes a MUCH longer time than anatomically modern humans).

In the most spoken language used by merfolk, there are many strange features:

1. There are no /k/ and /g/ consonants (almost all real-life human languages have at least one of those phonemes) (/k/ is the sound corresponding to English "crash", this word is also sometimes used in French) (/g/ is the sound corresponding to English "green", this sound is also found in French, as in graisse, which means "fat").
2. There are twelve standard vowels: the /a/ phoneme (as in French arbre, which mean "tree"), the phoneme corresponding to the "a" in English "madness", the /e/ sound (as in French étatisme, which means "statism"), the /o/ sound (as in French agneau, which means "lamb"), the /u/ sound (as in French ouvrier, which means both "worker"), the /i/ sound (as in English "hippie", this word is also used in French), the /y/ sound (as in French univers, which naturally means "universe"), the sound corresponding to the French "o" in ordinateur (which means "computer"), the sound corresponding to the "e" in English "red", the sound corresponding to the Japanese "u", the sound corresponding to "eu" in French euphémisme (which naturally means "euphemism"), and the schwa (as in English "about", or as in French petit, which means "petite", "little", and "small"). All vowels can be long. There are also the same nasal vowels as in French. There are four semivowels: the /w/ phoneme, as in English "world" (or as in French oiseau, which means "bird"), the /j/ sound, as in English "yellow" (or as in French hyène, which means "hyena"), the sound corresponding to the "u" in French fruit (which naturally means "fruit"), and the sound corresponding to the Japanese "w". Finally, there are five diphthongs: the sound corresponding to the "y" in English "fly" (or as in French chandail, which means "shirt"), the sound corresponding to French réveil (which means "awaking"), the sound corresponding to English "toy", the sound corresponding to English "brownie" (this word is also used in French), and the sound corresponding to English "slow".
3. There are the following consonants: /s/ (as in English "snake"), /z/ (as in English "zebra"), /f/ (as in English "fool"), /v/ (as in English "virus"), /h/ (as in English "heroine"), the sound corresponding to the English "sh" (as in "shower"), the sound corresponding to the French "j" as in joie (which means "joy"), the voiceless English th sound (as in "thick"), the voiced English th sound (as in "father"), the sound corresponding to the Dutch "w", the /p/ sound (as in English "parrot"), the /b/ sound (as in English "big"), the /t/ sound (as in English "transgender"), the /d/ sound (as in English "danger"), the /l/ sound (as in English "lesbian"), the /n/ sound (as in English "nerd"), and the /m/ sound (as in English "mother").
4. There are four rhotic sounds, in order from the most common to the rarest: the guttural "r" (as in French rédemption, which naturally means "redemption"), the sound corresponding to the English "r", the voiced alveolar trill (the Russian "r"), and the sound corresponding to the Japanese "r".

Also, merfolk mostly communicate by singing, because they are as solitary as blue whales, but they still CAN speak.

So, I wonder why would a language created by mammals from the Homo genus lack both the /k/ and the /g/ consonants?

Answer 1

Because it Just Happens Not To.

Hawai'ian has free variation between [t] and [k]. The more standard pronunciation, reflected in spelling, is [k], but you can replace all the [k]s with [t]s and it doesn't make any difference.

[t], it turns out, is the "default" plosive. If you test a bunch of people who speak different languages by asking them to identify phones, [p] and [k] are confused for [t] more often than [t] is confused for [p] or [k]. That is in tension with a tendency towards contrast-maximization--[p] and [k] are more distinct from each other than either is from [t]--but that just means there are in fact reasons for going either way. Hawai'ian happens to have drifted more in the [k] direction... but you just happen to have a language that happens to have settled on using [t] instead of [k].

Answer 2

Your list of phonemes doesn't include any closures at the velum. You could simply say one of the anatomical differences from modern humans is that they can't produce a full closure there. Maybe there's a ridge that makes it so, if the back of the tongue is pressed against the velum, some air can still get through.

You don't really need to justify this, though, especially when your creatures look more like belugas than modern humans. If they're that physically different from modern humans, I imagine most readers will immediately accept that their vocal tract is a little bit different too.