My guess based on what I read from the grandparent post is there was probably some poor product fit. Perhaps we would have done better marketing it towards "gringos"?

From your link:

> In July 2001, Dulce de Leche M&M's were introduced in five markets with large Hispanic populations: Los Angeles, San Diego, Miami, Florida; Mcallen-Brownsville, Texas; and San Antonio, Texas. The flavor never became popular with the Hispanic community, who preferred existing M&M's flavors, and it was discontinued in most areas by early 2003.

Perhaps there is an opportunity in introducing it to people who don't have previous experience with this flavor? Just thinking out loud...