In the nursery rhyme "Jack and Jill," why were the names Jack and Jill used?

While the true origins of the rhyme are unknown, there are several theories. The earliest publication of the lyrics was in the 1760's in John Newbery's Mother Goose's Melody. As a result, Jack and Jill are considered part of the canon of "Mother Goose" characters. As is common with nursery rhyme exegesis, complicated metaphors are often said to exist within the lyrics of Jack and Jill. Although these theories of meaning appear to make perfect sense, it does not follow that they are in fact the original meaning of the song. This is corroborated by the fact that the Newbery publication predates some of the common origin stories.
https://en.wikipedia.org/wiki/Jack_and_Jill_(song)

The roots of the story, or poem, of Jack and Jill are in France. Jack and Jill referred to are said to be King Louis XVI - Jack -who was beheaded (lost his crown) followed by his Queen Marie Antoinette - Jill - (who came tumbling after).
http://www.rhymes.org.uk/jack_and_jill.htm

Here is another theory on the origin of the rhyme:

"The name Jill has a very long history dating so far back that researchers are unable to pinpoint it's (sic) exact emersion into the world. It is believed to have started however as a form of currency. Back in the B.C. days a form of currency in circulation was called a 'gil.' It was kinda (sic) like our small change is today. Larger currency was known as a 'jack.' The story goes that the popular children's nursery rhyme, Jack and Jill, originated when a king and his economy were doing very well, the values of the jacks and gils were rising (Jack and Jill went up the hill). Unfortunately the people were struck by a plague that left everyone short of drinkable water (to fetch a pail of water) with the death of thousands the economy suffered (Jack fell down and broke his crown and Jill came tumbling after.)"

http://www.namespedia.com/index.php/Jill

Just from a poetic angle, if they wanted to write the poem as it was the names would have fitted afterwards just for rhyme, metre and alliteration.

Also many old poems were written as news stories before papers and there is a book on the origins of many. It was published last year and I've mentioned it before but can't find the name at the moment. This may well have been one of them.

It's a male-female version of Tom, Dick and Harry. (With similarities to John Doe and Richard Roe.) Generic names for a young couple. An old couple would be Darby and Joan. https://en.wikipedia.org/wiki/Darby_%26_Joan Read down to the end...

In "A Midsummer Night's Dream", Shakespeare seems to have introduced the relations of the names: "Jack shall have jill,/ naught shall go ill,/ the man shall have his mare again..."