That's maybe because the Left-Wing economic doctrines were vastly disproved in the last 30 years.
For exemple, in 1950 the Socialist School of Economics was perhaps the biggest one, even in the West(not in the US, but definately in Europe and Latin America).
Today the Socialist School is a joke, even the left-wing economist don't want to associate themselves with it.
The most "lefty" Economic School is the Neokeynesian one, and that's not left-wing by most standards. So truly left-wing Economics are dead.