According to the Oxford English Dictionary, cherub is a word that originated in the Hebrew, from which it came to Greek, then to Latin, to French, and finally from the French into English. In the French, the singular was cherubin and the plural was cherubins. In Wycliffe's 14th century translation of the Bible into English, cherubin was replaced with cherub, but cherubins was retained as the plural form of the word.
In the 16th century, acquaintance with the Hebrew led to the plural form of cherubins being replaced by cherubims. The later form is found only once in Coverdale's Bible but is consistently used in the Bishops Bible and the King James Bible of 1611. From around the beginning of the 17th century, scholars began to prefer the plural cherubim to cherubims (for example, John Milton used cherubim). Since that time, the plural form of cherubs has been replacing cherubim in many instances; cherubs being used as the individual ordinary plural and cherubim being used as the collective plural.
In short, the form cherubims is used in the King James Bible because of two reasons: 1) it was the historically correct form of the word in 1611; and, 2) it was based on a foreign form of the plural word, namely the cherubins of the French. I could not find as much information about the seraphims. It seems that seraphims was used because it's relationship to cherubims. Some of the historical quotations used the words together. It wold seem strange to have cherubims and seraphim. Therefore the "s" was added to make it seraphims. English is a language that has many historical markers. Your question concerns a couple of them.
