From Wikipedia for "Alpha":

>As the first letter of the alphabet, Alpha as a Greek numeral came to represent the number 1. Therefore, Alpha, both as a symbol and term, is used to refer to the "first", or "primary", or "principal" (most significant) occurrence or status of a thing.

Since they were the first "verse" to make contact with another, they became the "Alpha Verse". Same reason games/software are first "released" in an alpha-state.