The 2JZ, which is based on a design from thirty years ago, has been out of production for thirteen years.
Toyota has discontinued the 2JZ engine in favour of the B58 due to increased fuel efficiency, decreased weight, improved emissions and a newer design. The 2JZ is over-engineered and heavy in comparison to most modern engines.
Several reasons exist as to why the 2JZ has been discontinued, including the following.
Tooling / cost
Toyota may no longer have the equipment, or if they do they may have repurposed the engine's manufacturing line for another engine, or they may not even have the factory any more.
It is substantially heavier and over-engineered compared to the B58 and produces significantly less power in its stock form. The B58 is made from aluminium and the 2JZ is manufactured from iron.
The 2JZ is likewise unlikely to have cleared emissions restrictions and is typically less fuel efficient than newer more modern engines, like the new B58.