Why was the Spitfire's elliptical wing almost uncopied by other aircraft of World War 2?
Interestingly, I couldn't find an answer to this question on the website, but I've found an answer of Peter Kämpf on Quora. He brings forth the same arguments I wanted to mention, so I'll repeat them here.

Elliptical wings are very good - aerodynamically. If you want to minimize induced drag for a given lift requirement, you end up with an elliptical wing. 

But a plane is not only aerodynamics. 
You also have to consider:


Weight, an elliptical wing is not structurally efficient, and will lead to a higher weight, which leads to higher lift requirements which will lead to more induced drag, even with a very efficient wing.
Controllability, where and how the wing stalls determines if you're able to recover from a stall. Elliptical wings stall tip first, leading to bad stall behavior.
Manufacturability, a fully elliptical wing is very hard to make, with its double curves. This will make the wing more expensive.


If you include these factors, you'll see that you'll end up with a compromise. If you use wing taper (which is somewhat close to the aerodynamic optimal shape) but much easier to make and much lighter you'll see that you'll end up with a better design overall.

An analysis of how the design of a wing changes if you include the structural requirements was done by Jones and can be found here.