Ford used to own ~30% of Mazda before selling the shares back to Japanese entities. Because of that, they were able to share engines and platforms. For example,

Ford used to own ~30% of Mazda before selling the shares back to Japanese entities. Because of that, they were able to share engines and platforms. For example, the Mazda 3 and Ford Focus shared the Ford C1 platform and Mazda MZR engine. The Mazda 6 and Ford Fusion shared the Mazda G platform and MZR/Duratec engines. The Mazda 2 and Ford Fiesta shared the Mazda D platform. Since the breakup, Mazda has moved onto their newer Skyactiv engine/platforms, but lost access to the Ford Duratec V6 and C1-platform (the current Mazda3 sits on top of a Miata derived platform I believe). Ford is still using the older MZR designs as a basis for their EcoBoost I4 engines, but lost access to the Mazda G and D platforms underpinning the Fusion/Fiesta (and the Volvo derived D3 platform for the Taurus). I guess they decided to discontinue those lines instead of developing a new platform/engine for them. The C1 platform was a joint Mazda/Volvo/Ford venture, Ford may have decided that it's too expensive to develop an update to it on their own. Once you remove all vehicles that use the MZR-I4s and non-Ford platforms, the only car you're left with is the Mustang.