According the some recent studies highlighted in this article, it may have to do with the airplane itself:
"The researchers fed 48 blindfolded participants a variety of foods from biscuits to rice crackers to cheddar cheese. At the same time, headphones either canceled out noise or provided various levels of white noise. The subjects then rated the intensity of the flavors and how much they liked or didn't like them.
The result: the higher the noise level, the less the participants tasted salty or sweet flavors. Their sensitivity to the crunchiness of their food was amplified."
Of course, some restaurants on the ground are plenty noisy but still tasty, so what gives? Read the article for more...