Because one is more parsimonious than two.
"It is superfluous to suppose that what can be accounted for by a few principles has been produced by many." - St. Thomas Aquinas.
"Among competing hypotheses, the one with the fewest assumptions should be selected." - William of Ockham
Since I provided to you a model of the Universe that does not ask for a cause, I would say that zero is also more parsimonous than one.
Ciao
- viole