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I don't think it's entirely a matter of "need". All these implementations are independent projects.

Some of them include their own features on top of the standard. Some of them focus on specific use cases and/or optimize for specific performance characteristics. Some of them are toy projects.

How many C implementations/compilers are there? Even Python (which has a massive and often ill-defined spec) has multiple implementations.

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> How many C implementations/compilers are there?

I can compile same source code with pretty much any C compiler. I might be wrong, but the lisp compilers are all "slightly different". I guess scheme ones can be the same? But are they really?

That's fair, until you get into GCC extensions. I guess the difference is that the Scheme "core" is typically smaller and the amount of extensions is typically larger.

See also: Common Lisp, which has a similar problem.

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