The "la" was not dropped, as it was originally a name that originated with the Hebrews/Phoenicians, who intermingled a lot. "La" was used by speakers of Latin languages, because that's how their languages work.
There is evidence to suggest that the Phoenicians associated the start Merica with the American continent. Josephus, the ancient Jewish historian, wrote, "The Essenes believed that good souls have their habitation beyond the ocean, in a region that is neither oppressed with storms or rain or snow nor with intense heat, but refreshed by the gentle breathing of the west wind which perpetually blows from the ocean. This idyllic land across the sea to the west (or sometimes to the north) is a common belief to many cultures, from the Jews to the Greeks to the Celts. The Mandaeans, [direct descendants of the Nasoreans, who Knight and Lomas suggest were the residents of the Qumran community where the Dead Sea Scrolls came from] however, believe that the inhabitants of this far land are so pure that mortal eyes will not see them and that this place is marked by a star, the name of which is 'Merica' "