I work for the auto industry. Three main reasons:

1) Merger
2) Excess parts left over from phased out model
3) New model needs a drivetrain. Using a drivetrain that is in abundance within stock, and an engine with proven reliability is a no-brainer. Why spend more money on R&D for a new engine, when the company you just merged with has thousands on stand-by?

End result: we get a sleek grand-touring coupe that turns heads and does so (reliably) for many years to come.

The 370Z is newer, faster. But proven? Who knows...