Given the history of CO/UA at EZE, I can't say I'm surprised. Using award tickets, I was actually booked northbound on that flight for November, but cancelled when Kirchner would no longer allow my family to enter the country in Mendoza fee-free (we're headed to South Africa instead).
Which I think is the root of the problem: Argentina isn't a terribly attractive country for North American tourists now, and it certainly isn't a terribly attractive destination for business, either (can you say "rule of law").
So no surprise that a marginal deep South American player like UA decided to leave.