Why can't humans use dietary citric or acetic acid as a primary source of energy?
Why can't eating citric acid be used as a temporary primary source of energy - in place of fat/carbohydrate/protein?


No reason it can't. Citric Acid technically contains 2.5kcal/g -- which is almost half of protein/carbohydrates, almost a third of alcohol, and a fourth of fat.

So if someone needed 2500kcal/day, they'd need 1kg of straight Citric Acid. There's about 47g/L of Citric Acid in citrus fruit juice. So if you wanted it all from "natural" sources you'd need to drink 21.3L of your favorite citrus juice, though that's a bit comical.

In terms of being more efficient since Citric Acid bypasses the first two enzymatic reactions,the part where Citric Acid is processed into alpha-ketoglutarate via Isocitrate Dehydrogenase is the rate limiting step of the entire cycle. You wouldn't be saving any time by flooding your system just before the bottleneck. 

Interesting question, though!