The roots of using 'Metroplex' to describe the whole Dallas–Fort Worth region are obscure. Some evidence links the term to the early 1970s when the two cities were joining together to build DFW airport. Boosters wanted something snazzy that promoted the entire region.
Regardless, the word stuck, sort of. While you'll hear it used around the Metroplex, it's not a word locals use outside the region. Rare is the local who says they are from the Metroplex. Texas Monthly summed up the term as a 'grotesque word that means nothing.'