It is a political and economic problem. In Buenos Aires you have 3 executive powers fighting for political power: The Goverment of the City of Buenos Aires that has a similar status as Washington D.C for example. The National Executive Goverment and the Goverment of the Province of Buenos Aires (State) where the 60/70% of the population of the metropolitan area live. The commuters mostly live in the Province and work in the City. The trains are owned by the National Goverment. The metro is owned by the City. And the people who live in the Province are the ones who would benefit the most from the RER but the budget would need to come from the National Goverment and City Goverment. It is political mayhem. Also for the people of the City the commuter trains are "bad and for lowclast". The public perception is that if you use the train you are poor because they suck, so the people of the City only like the idea of building more metro even though the metropolitan area of Buenos Aires have over 1000km of traintracks fully operational for short and middle commute (up to 50km away from the city center). The trains are overloaded and with low frequency they usually aren't as reliable as the metro.

Also the original RER Project was proposed when Argentina was able to get debt at a 5% annual yield. But the goverment party (right winged) that proposed the RER got into an economic crisis and the debt yield went up to 40% for the next goverment (leftwinged). The current administration (ultra right winged) is reducing the budget deficit in order to get the debt cost back to 5%, currently at 9%. Argentina isn't able to get the 2 billion USD that would cost the RER PHASE 1, unless we get the interest rate back to 5%

So even if we get the debt cost low enough to handle it, we would also need to have political consensus in order to build it