It was from mid 80s until 2015. In the early days, living was pretty poor and having a stash of food was neccesary. Later, since 00s, it was probably more of a psychological inertia to continue, although planted volumes were reduced quite a bit.

I think your math is skewed to a higher end because there potato is (and probably was) significantly cheaper than in US or EU (today it is ~4 times cheaper, 0.7$/kg).

Even though, considering todays mean salary there (5$/hr), 2$/hr is not that bad for such job...