The interracial couples stem mostly from an advertising philosophy that the potential consumers should be able to see themselves in the ads because it will make them relate to the product more. It's what let to ads to continue being overwhelmingly white even as true racism subsided. Basically if 80% of the potential market is white then using a white actor will allow target your ads more effectively to 80% of the population. Eventually advertisers started thinking that using interracial couples will allow them to market to an even broader audience. i.e. If 80% of the market is white and 10% of the market is Asian, then using a white-Asian interracial couple will allow them to market to 90% of the population.

It's not a terrible idea from a business standpoint although it does contain certain questionable assumptions. Will an ad featuring a white man and an Asian woman appeal to an Asian man because he's able to see an Asian person and a man in the same ad? If not instead of getting 90% of the market you're getting half the white market and half the Asian market (45% of the total based on these fictional numbers). Additionally white men and Asian woman may not identify with the idea of being in an interracial relationship so you may see further drop off of those numbers. It's the same basic idea of why an ad featuring a heterosexual couple may not appeal to gay men simply because one of the people in the ad is a man.