Overriding BS5's color map with CSS rule

Question

Related to Overriding Bootstrap 5 colors doesn't change btn color classes, why is it that CSS rules cannot override colors defined in the BS5 color map?

For Example the following custom CSS rule sheet won't override the $primary color in the BS5:

@import "bootstrap";

.bg-primary{
    background-color: yellow;
}

Instead the way to change color of any classes with .bg-primary is:

$primary: yellow;