For two reasons:
The primary one, of course, is that energy and mining interests are licking their chops at the prospect of Argentina having to privatize its state energy concerns (mainly YPF) - as well as getting preferential exports terms (a hyper-devalued currency, lack of export audits, etc).
But some are also looking forward to the same trend you saw after 1989 from Eastern Europe: the massive influx of a largely white population - which they believe can be persuaded to vote Republican later on (though around 70% of Argentine-Americans did vote for Hillary in 2016).
Now - Argentines aren't as overwhelmingly white as Eastern Europeans. But compared to most other Latin immigrants, it's usually a noticeable difference in that regard.
In any case, it would be a bad a idea for a U.S. administration of either party to be too chummy with the unbalanced Señor Milei - because when his Bush-like policies lead to a Bush-like collapse (but worse, b/c they don't have a Fed to bail them out), Milei would only undermine U.S. international standing if they were perceived to be propping him up.