The number of four-year degrees awarded each year in the United States has more than doubled since 1970, so the supply of high-skilled labor has shifted right, from S₀ to S₁. On its own that shift would have lowered the wage for high-skilled work.
Instead the gap between college and high school earnings widened: workers aged 25 to 34 with degrees earned about 1.85 times as much as high school graduates in 2020, up from 1.59 times in 1995. For the wage to rise while supply was growing, demand must have shifted right by more, from D₀ to D₁. Technology that raises what skilled workers can produce, and global markets that let the best of them sell to far more customers, are the usual explanations. The sliders move each curve separately, so you can see what each one does on its own.