Before going to far, it's important to note that your premise isn't entirely correct: nutritional standards are currently created by a joint effort of the Department of Agriculture and the Department of Health and Human Services (HHS). [Source: History of Dietary Guidelines for Americans, Health.gov]
This has been true since 1980.
Shortest Answer: It's their Mission
The USDA's espoused mission is:
We provide leadership on food, agriculture, natural resources, rural development, nutrition, and related issues based on public policy, the best available science, and effective management.
Source: About the USDA, USDA.Gov
At the simplest level, the USDA deals with nutrition policy because they are charged with providing policy leadership on issues related to food. It's within their umbrella.
Why was USDA Involved in Nutrition Standards?
USDA has been publishing nutritional standards since 1894 [Source: Dietary Recommendations and How They Have Changed Over Time, USDA]. The Center for Disease Control wasn't organized until 1949, and the Department of Education wasn't organized until 1979.
The first nutritional standards were published by the USDA before nutrition was as formalized as today. In fact, W.O. Atwater (who USDA credits with the first publication) was a chemist with an expertise in nutrition. At the time, he was Director of an unrelated office at USDA. The first standards were published the Farmers' Bulletins, a series of publications on agricultural subjects intended for consumption by farmers.
It seems that the USDA became involved in nutrition for at least two reasons, then:
- Prior to nutrition becoming its own discipline, it was the domain of chemists and other scientists with a knowledge of food - which the USDA had.
- Nutrition is related to agriculture, in that both involve food (either as production or consumption)
Another View: Interest Group Theory
In political science, interest group theory (briefly) hypothesizes that interest groups band together to lobby for benefits from the government. Regulation is how the government provides benefits to interest groups.
The Department of Agriculture was made a cabinet-level department based on the lobbying of farmers' unions in the 1890s.
At a theoretical level, the department exists for the purpose of passing federal benefits to farmers. So it seems sensible (although again, entirely theoretical) that this is why the USDA handles nutrition guidelines: it serves as a kind of marketing tool for American products. Some alleged that this is why grains were on the bottom instead of fruits and vegetables.