Probably because they could be turned out more easily on a lathe, the short upright columns for supporting a handrail were made, in the Middle Ages, in circular section and a doubly curved outline, a narrow neck above, swelling into a pear-shaped bulge below.
These columns strongly resembled the shape of the flower of the wild pomegranate. The Italian name of this flower, taken from the Latin and thence from the Greek, is balaustra, and this then became the Italian name of the column.
French architects adopted the column because of its beauty, but spelled the name balustre, which, in turn, became baluster in English. Thanks to careless speech and to indifferent hearing, the form banister began to be used shortly after the new word baluster was introduced.
The corrupted term has been frowned upon and condemned by purists ever since, but it has taken a firm position in the language and is now more commonly used than baluster.
“Over the banister leans a face,” sang the poet, little knowing perhaps, the true poetry of his song.
Banister, you see, is a careless mispronunciation of baluster, introduced about three hundred years ago.
And baluster, the proper word, was formed from Greek balaustion, which was originally the term for a specific flower.
Our poet thus sang, in effect, “Over the flower of the wild pomegranate leans a face, Tenderly soft and beguiling.”
Greek architects used the outlines of these flowers, doubly curved, slender above and bulging below, for series of short pillars supporting handrails or copings, thus giving us the modern sense.