At the risk of getting too basic, I think one of the many issues that Epicurus was concerned about was the evidence implications of "Life exists only on earth."
Much like "The earth is the center of the universe," if either or both of those are true, then there is clearly something special about the earth, and that something special would imply a supernatural explanation.
I therefore think that it was important to Epicurus to make the obvious point that there is never only one thing of a kind here on earth, and therefore (if the universe is boundless and eternal, as he had elsewhere given his argument) then life will exist throughout the universe, and not just here.
Once the existence of life throughout the universe is established through "never one of a kind" then you apply the "isonomia" that things also exist in a progression from "lower" to higher" in terms of complexity, and you arrive fairly easily at the conclusion that there are beings in the universe who do not die, and don't work all day to pay taxes.
I personally am satisfied that even if Epicurus' argument is ultimately no more complex than this, that it is compelling.