Because an HSL value translated directly to sRGB will look different when translated to P3. The color spaces themselves define different ranges of R, G, and B. The only backwards-compatible method to derive P3 colors from HSL values would be with saturation and lightness values greater than one.

HSL is not really abstracted. Instead, it is just a different way of specifying RGB values. So you can't have HSL switch to the P3 color space, because you would end up with color shifts.

In other words, same HSL => same RGB, and same RGB but different space => often a different color.