Why do Argentinians use "vos" instead of "tú" in informal speech?

pronominal: only the pronoun vos is based on the Latin plural, while the verbal form is that of the Latin second person singular: vos cantas, vos quieres.

Voseo means that the allocution of the second person singular is not based on Latin TŪ ‘you_{sg.}’, but on VŌS ‘you_{pl.}’. Voseo can appear in three different forms:

As for the question of why a certain system is habitualised in a certain variant of the language, I do not know if it is possible to give exact explanations. In fact, the allocutive system is generally very varied, since it is not only a question of grammar sensu stricto, but is also part of pragmatics; that is, social or even political issues also contribute to its evolution. For example:

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complete: both the pronoun and the verb are based on the Latin plural forms, e.g. vos cantás ‘you sing’, vos querés ‘you want’.

The underlying process closely resembles the one that led to the replacement of thou art and thou hast with you are and you have in modern English.

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Subsequently, in European Spanish, the opposition tú—usted (2nd person singular intimate—distant), vosotros—ustedes (2nd person plural intimate—distant) was consolidated, also by grammatical prescription. American Spanish, however, lacked such instances defining clear linguistic rules, so that regional variants were “fed” by the manifold inventory containing both tuteo and voseo, which could develop freely and far from normative influence, just as the corresponding verbal forms could develop naturally. Besides, in Latin America, ustedes ‘you_{pl.}’ is the only second-person-plural pronoun; that is, there is no vosotros in America.

The allocutive or pronominal system of American Spanish is very varied. You don’t only have voseo (which can be complete, pronominal and verbal), but there are also ustedeo, tuteo¹, as well as tripartite systems. The distribution of these systems varies according to region and register. In fact, also the use of voseo is not limited to Argentinian Spanish.

second person singular distance: usted.

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Tuteo means that both the pronoun and the verbal form go back to the Latin second person singular.

That is, the allocutive or pronominal system tends to be unstable and shows substantial differences also between related languages or between different dialects, also because the track of grammaticalisation is quite “open”. Therefore, it is not surprising that also in American Spanish there are several realisations of the allocutive system, even though I think it is difficult to verify why a certain system developed precisely in a certain place.

Romanian features a highly complex allocutive system that includes, at least virtually, four different address pronouns for the second person singular: tu, dumneata, dumneavoastră and domnia voastră (which, however, is hardly used any more, if not in an ironic function) and also three degrees of distance for the second person plural.

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second person plural distance: ustedes.

In Brazilian Portuguese, você was conventionalised as a second-person-singular intimate pronoun. That is, in a certain way, in Brazil prevails what in the Spanish context is called ustedeo.

second person singular intimate: tú ~ vos.

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Ustedeo means that the original distance pronoun usted is used in intimate contexts, so it has the same function as tú in European Spanish and of vos in Argentinian Spanish.

In late Latin (Imperial and Christian period), speakers began to distinguish between a distance (or “polite”) form and an intimate (or “informal”) form. In the distance form, i.e., when addressing a person “of respect”, the pronoun vōs was used, whose main meaning was that of the second person plural (‘you, ye’). This situation was still retained in Old Spanish: tú was the intimate pronoun of the second singular and vos was the distance pronoun of the second singular and the pronoun of the second person plural. Later, however, the function of the distance vos moved closer to the function of the intimate tú, until in the “Siglo de Oro” (~1550–1660), tú and vos were almost synonymous (for instance, in Tirso de Molina in whose opus it is difficult to associate tú and vos with different functions). For this reason, the second-person-singular form vos, which didn’t mark distance any more, was often replaced by the formulation vuestra merced ‘Your Grace’, whose contracted form usted was continuously habitualised as the new second-person-singular distance form. Likewise, the plural pronoun vos was replaced by vosotros (literally: ‘you others’), so that the only remaining function of vos was that of a second-person-singular intimate pronoun, alongside tú:

The Italian distance pronoun is lei ‘~she’. In this case, it was not a nominal syntagm to be conventionalised (like vuestra merced > usted in Spanish), but the reference to it; i.e. lei ‘she’ refers to vossignoria < vostra signoria ‘your lordship’. In the dialects, however, the use of voi (< VŌS) prevails, and for instance in the colloquial Italian of Naples there is also a tripartite system (tu, voi, lei).

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second person plural intimate: vosotros.

verbal: only the verbal forms are based on the original plural: tú cantás, tú querés.