I think poverty and lack of know-how or recognition of good quality has a lot to do with it.
The other part is fickleness and capriciousness that runs rampant in this society. Taste over health. Ease over work. Cheap over expensive. Why spend good money on a beautiful piece of fish that tastes rather bland, when you can spend a lot less on something that goes down a lot easier?
I for one don't mind simple dishes - too many foods in other places gets overtaken by spices, sauces, and strong flavor ingredients that mask the quality of the underlying meat. But a little effort is appreciated. The differences between Spain and here is astounding... I keep being told that the restaurants and cuisine here have Spanish influence (and it does in the most superficial ways) but there's really no comparison. In Spain you can buy excellent fish in even the smallest towns way up in the mountains away from the sea. Here in Buenos Aires, living right next to the sea, you have to really search. In my area, there isn't one pescaderia that I know of, and even if there was, I wouldn't trust it. The Spanish also have a sort of obsession with food quality and health - at least my mother always did. Having grown up on a farm, maybe they didn't have money to buy clothes or other things, but they ate well. Lots of fruit, vegetables, salad, potajes (garbanzos, lentejas, etc.) all your typical healthy peasant dishes. Here peasants didn't really develop in the same way with land and farming. Yea, you had some, especially in the interior -- but poor people were more poor and Buenos Aires doesn't seem to have the same culture.
I don't want to speak too badly of society here because Argentina (and Argentines) has a lot of redeeming qualities... but I think this blindness to quality has a lot to do with the immaturity of the people. Look at the politicians and who gets in power... the most recognized stars (Mirtha Legrand for example... would she be seen the same way elsewhere?)... the obsession over a sport as the most important thing... it's rather adolescent. Argentina is a young country, but not that much younger than the US (obviously with very different beginnings and history.) The people here have been stunted in growth by all the awful governments and poverty that has plagued them.