Why does the IPA use four main vowel heights?

Question

Because vowels exist at infinitely precise points on large acoustic and articulatory spectrums (vowel spaces), the study of phonetics uses generalized waypoints to describe them. The International Phonetic Alphabet includes 28 distinct symbols for these. 22 of these are placed at one of four main heights (close, close-mid, open-mid, open) and three main backnesses (front, central, back). Six of the symbols are at extra places (near-close, mid, near-open, near-front, and near-back) that don't get gridlines on the chart.

I believe a lot of the IPA vowel system stems back to Daniel Jones, who promulgated a system of eight primary cardinal vowels (i, e, ɛ, a, ɑ, ɔ, o, and u) that he felt could describe a significant fraction of the vowel sounds in the world's languages. These were also at four heights.

However, it's well accepted (I think) that a huge portion of languages utilize vowels at a mid-height, which is absent from Daniel Jones' chart and is only used for [ə] on the IPA. Phonologies with five vowel qualities are extremely common in world languages (Spanish, Japanese etc.), and these often include mid-height peripheral vowels that require symbols with diacritics ([e̞] and [o̞] or [ɛ̝] and [ɔ̝]) to be precisely described by the IPA. Some prominent European languages (French and Italian) do have their peripheral vowels at four heights. The IPA chart is perfectly adept to describe these phonologies with simplicity, but not most others.

Essentially, it seems like the languages with four vowel heights won, and the myriad of languages with mid-height vowels were forgotten in a ditch on the side of Phonetics Avenue. Why is this? Couldn't vowel charts accomodate all the different phonologies much more easily by using five heights? These could be close, mid, and open to describe systems with five qualities or fewer, plus two extra heights (close-mid and open-mid) for the French- and Italian-type systems.

Answer 1

Answer to "Why" is original heavy bias of IPA towards English, French and German languages, which have so many vowel heights, and so many vowels.

Special symbols /ɪ/, /ʏ/ and /ʊ/ for vowels outside "main" 4 heights were also included because English and German use them. E.g. vowel /ɪ/ could be alternatively written as /i/ with down-arrow diacritic and minus diacritic.

International Phonetic Alphabet:

In 1886, a group of French and British language teachers, led by the French linguist Paul Passy, formed what would be known from 1897 onwards as the International Phonetic Association (in French, l'Association phonétique internationale). ...

History of the International Phonetic Alphabet:

The International Phonetic Association was founded in Paris in 1886 under the name Dhi Fonètik Tîtcerz' Asóciécon (The Phonetic Teachers' Association), a development of L'Association phonétique des professeurs d'Anglais ("The English Teachers' Phonetic Association"), to promote an international phonetic alphabet, designed primarily for English, French, and German, for use in schools to facilitate acquiring foreign pronunciation. ...

Answer 2

The simplest answer is that there's no harm in omitting symbols that aren't relevant to a particular language. Most descriptions of languages like Spanish and Swahili simply use either e o or ɛ ɔ for the mid vowels, disregarding the other. If there's no contrast between, say, e and ɛ in a particular language, it's likely that the mid front vowel (whether you call it /e/ or /ɛ/) extends across the range of both symbols. And if you're trying to transcribe particular samples in great detail, you'll probably still need diacritics, even if you did have an extra symbol for a truly mid vowel.

Some extensions to the IPA, like Canipari's "CanIPA", add additional symbols for this purpose. But they haven't really caught on, because most people are happy to simply use e o or ɛ ɔ and ignore the other pair.

(Some other transcription systems, like Uralicist notation, only have symbols for three heights rather than four. This is also a reasonable way to do it, but means you need to add diacritics as soon as you go past six vowels. That's why the IPA didn't go this route.)