I don't think it's discrimination, it's more statistics.

If we consider the demographics of the United States for example, there are around 230 million white people, and just 39 million black people. [1] Knowing this, it's clear that there is a larger market for those with a white skin tone, and thus the lighter skin tones would sell better, the increased sales of the lighter skin tones allows for more investment into different shades of the white skin tone makeup.

Loreal would be appealing to the "masses", rather than focusing on a more niche area, as this allows them to make more money. If they were to increase the diversity of the darker skin tones, this could impair the income of each individual darker shade.

The increased diversity would result in the profit for each individual shade becoming lower, and this would affect retailers. The retailers wouldn't want to stock 12 different shades of black (with each shade costing them money), if they know that 6 of those shades are barely making money, and taking up shelf space for the lighter shades which are making more money.

Eventually, this would result in Loreal discontinuing those newer shades, as retailers would stop buying them, limiting to the original darker shades which were more popular.

This can be seen in the fast food industry. Once upon a time, McDonalds launched a hot dog. They're pretty much the most popular fast food restaurant in the world, so you'd think they'd have no issue selling hot dogs, right? No, they ended up discontinuing it because not enough people were buying them.

In this case, Loreal is like McDonalds, as much as they could add more items, the demand is not there. By adding the new shades, they would have to convince retailers to add those shades to their stock, but they may believe based on current sales of their existing darker shades, that such items would be low interest and result in not enough demand to continue selling them.

[1] https://en.wikipedia.org/wiki/Demography_of_the_United_States