In the case of Elizabeth, it was likely just so common at one time that many variants were needed. Also Mary (you mightn't think there are many variants of Mary, but, for instance, Polly is a variant of Mary. Figure that one out.) At one time, pretty much every family in Ireland had at _least_ one child called Mary. Sometimes more than one. This ultimately resulted in Ireland having two consecutive presidents called Mary (and in the 2011 election, which lead to Ireland's first non-Mary president in 20 years, two of the candidates were also called Mary or variants thereof). The Marys have now largely aged out, and there were no candidates called Mary at all in the most recent election.

And as memorialized in a movie and TV series, Buffy.

and Bess