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.