Japanese poets count on (or onji), a sound represented by one kana; the closest English equivalent to this is a syllable. Thus, the 'syllable' count is a function of the Japanese language, in that Japanese is written in kanji (Chinese characters-which can contain one or more on) and kana (Japanese phonetic script). There are also on, called 'particles' in English, that serve a precise function in Japanese, but that have no English equivalent; these are used in haiku to create a pause, provide emphasis, or sometimes simply to allow for the proper number of on; some poems achieve the desired emphasis by adding an extra on, or by subtracting one.
The origin of the 5-7-5 pattern stems from the one consonant, one vowel, structure of all but six of the Japanese on, and a belief among the Japanese in the melodic rhythm this arrangement creates. This closely approximates the effect of rhyming in some English poetry, but the rhythm of Japanese haiku cannot be duplicated in English.
Furthermore, in Japanese, 'restricting' the poem to 17 on creates a particular impact on the poem (similar to the impact the rules of a sonnet creates). Japanese haiku are very sparse, written in a rarefied, even abbreviated, form of Japanese (most haiku would require many more on if written in standard Japanese), the idea being to say 'just enough' to allow the reader to enter the poem.
In English, insisting on 17 syllables usually has the opposite effect because it forces the poet to say more than is required to convey the moment or image. In fact, English haiku that insist on the 17 syllables arranged strictly into three lines of 5-7-5 syllables respectively are often wordy and unwieldy. I have read many potentially good haiku that have been made less so by being 'padded' with extra syllables.
Some poets argue that the 'discipline' imposed by the syllable count allows them to create better poetry, but for the most part, I've seen that the opposite is the case; a much more useful convention/restriction would be to use as few words as possible.
This prescription is also not a rule. Rules are established by governing bodies and no such organization exists for English haiku, though the first substantial non-Japanese haiku organization is the Haiku Society of America. In response to this issue, and in an effort to correct definitions that began to appear in English-language dictionaries, they wrote:
The definition of haiku has been made more difficult by the fact that many uninformed persons have considered it to be a "form" like a sonnet or triolet (17 syllables divided 5, 7, and 5). That it is not simply a "form" is amply demonstrated by the fact that the Japanese differentiate haiku from senryu──a type of verse (or poem) that has exactly the same "form" as haiku but differs in content from it. Actually, there is no rigid "form" for Japanese haiku. Seventeen Japanese on (sound-symbols) is the norm, but some 5% of "classical" haiku depart from it, and so do a still greater percentage of "modern" Japanese haiku. To the Japanese and to American haiku poets, it is the content and not the form alone that makes a haiku.
Furthermore, Cor Van Den Heuvel, a well-known haijin and translator of Japanese poetry in 1987 wrote:
"A haiku is not just a pretty picture in three lines of 5-7-5 syllables each. In fact, most haiku in English are not written in 5-7-5 syllables at all--many are not even written in three lines. What distinguishes a haiku is concision, perception and awareness--not a set number of syllables. A haiku is a short poem recording the essence of a moment keenly perceived in which Nature is linked to human nature. . . The poem is refined into a touchstone of suggestiveness. In the mind of an aware reader it opens again into an image that is immediate and palpable, and pulsing with that delight of the senses that carries a conviction of one's unity with all of existence."