Other answers so far are incorrect.

Legumes, and few other types of plants are categorized as Nitrogen Fixers.

Nitrogen is one of the elements required to make protein, and not necessary in fats or carbs

A nitrogen fixing plant is one that has a symbiotic relationship with one of a few specific types of bacteria. These bacteria make colonies attached the roots of these plants (shown in the link). The bacteria take nitrogen from the atmosphere (gas) and turn it into a form of nitrogen fertilizer that plants can use (liquid).

Legumes, and other N-Fixers essentially get free fertilizer, where the rest of the plant kingdom does not. These N-fixers have a surplus of nitrogen compared to other plants, so they can use it their seeds, assumingly as a form of energy storage, instead of relying on carbs or fats to store energy in the seed.

In fact, Legumes and other nitrogen fixers are a component to regenerative, sustainable agriculture, because they can produce nitrogen naturally, instead of us making N fertilizer out of fossil fuels.

tl;dr legumes and their bacteria buddies get free nitrogen, which is needed to make protein, while almost all other plants have to struggle to get nitrogen so they use oils and carbs to store energy in the seed instead.