TL: DR there is no reason except not being able to afford Arma 3 to play Argo.
Argo looks exactly like Arma, plays like Arma and even feels like Arma.
The problem is Arma is built upon human interaction and customer server modifications, Argo lacks both of these quantities by a full mile and then some.
Argo is not a bad game, it is just an empty and feature incomplete game, the sample sized piece of chocolate cake that leaves you wanting more, Arma 3 is that more.
If you like anything about Argo, Arma 3 does it and often does it better.
With almost no community and sparse updates, Argo is a waste of bandwidth to download, wait for a sale and grab Arma 3 instead.
24 Hour Peak CCU Comparison
Arma 3: 17.317