I have repeatedly brought the point up that in decades past, when it was routine for top-class 2YOS to grow up to be classic horses at 3 and still be around at 4, the richest races in the country were for 2YOs.
Although the trend toward big money for 2YO races began in the late 1800s, it's very apparent by the 1950s; consider that Tom Fool, best remembered now for winning the Handicap Triple Crown and going undefeated in 10 starts at 4 in 1953 (including the Met Mile, Suburban, Brooklyn, Whitney, Pim Special, Carter and assorted other races) en route to HOY honors over Native Dancer that year, was a champion at 2 and never again took home a purse more than half of what he won in the Futurity. By the mid-1960s, six or seven of the most valuable races of the year were for 2YOs. This persisted into the 1970s - the richest payday of Secretariat's life came not in the Derby, Preakness, Belmont or Marlboro Cup, but as a 2YO in the final running of the Garden State Stakes.

Actually, purses for top-level 2YO stakes races in the US have not been as low(compared to similarly prestigious stakes for older horses) as they are now in an enormously long time - most probably since the 1800s.

The problem lies elsewhere.