A big reason, which the Eat24 article sort of delves into, is use case. If most porn viewers are watching between 7pm and 3am, those are fantastic hours for a food delivery service; they might be shitty hours for a lot of other products or services. (Though I'd imagine they would make great hours, and porn great ad inventory, for impulse-buy products).

As others have mentioned, brand association is another big factor. Probably the biggest factor. Most mainstream companies are extremely (ahem) anal about where their brands pop up.