My theories-
Spanish immigrants arrived first, and Argentina received Spanish-seaking spillovers from nearby colonies.
OR
Spanish immigrants who more unified in their spoken dialect, while Italian immigrants (coming from both the North and the South of Italy) were very disunified in their dialects, which made Spanish the most reliable lingua franca.
I don't get your question. Argentina is a former Spanish colony who declared independence in 1816 and, hereditarily, speaks Spanish.
Italians emigrates to the country between the second half of XIX and the first half of XX, in the same way that they emigrates to the US and others countries of America. The particular issue that occurs in Argentina is that the amount that Italians arrives to the country overlaps the current population.
However, the government and the society educates the migrants to adopt Spanish as their language. But Italians won't give it that easy and they heavily influenced the Spanish that is being speaked in Argentina, not only by accent but also in the vocabulary, creating new words that are a mix from Spanish and Italian languages.
That is how Rioplatense Spanish and the Voceo is born.