The BT-50 was co-developed with Ford's Australian division. In their current iteration, the BT-50 and its Ranger twin were never on sale in the United States. Mazda's last pickup in the US was the B-Series, itself also a rebadged version of the old Ford Ranger.
Isuzu pulled out of the US passenger vehicle market in 2009, although it still sells commercial vehicles here. Isuzu, continuing a long and complicated relationship with General Motors, co-developed the diesel engine that currently powers the Chevrolet Colorado and GMC Canyon. In select markets, Isuzu sells a rebadged version of the Colorado as the D-Max.
Details on the Mazda/Isuzu tie-up are scarce. A single press release simply says "Isuzu will produce next-generation pickup trucks for Mazda, based on Isuzu's pick-up truck model." This isn't the first truck collaboration between the two companies. Since 2004, Mazda has sold the Titan, a rebadged version of the Isuzu Elf commercial truck. The Elf is sold in the US as the Isuzu N-Series.
Those in the US hoping for a new competitor in the midsize-truck segment shouldn't get their hopes up for the new truck. There are currently no plans to bring them here. There is hope, though, that Ford will bring the Ranger replacement stateside sometime in the next few years.