I think it is because "Lisp" is such a broad description of a language. The only common thing between all the lisps that I know is most things are in brackets, and uses prefix function notation. Eg
(fun (+ 3 4))
However nearly everything else can vary between implementations. Scheme and CL are completely different languages, and should be considered like that.
I think calling the lisp community fragmented is like calling the "C like" community fragmented. It has c,c++,d,java,c#, go, javascript, python and many other languages which I can't think of.
In summary: Lisp is more of a language property (like garbage collection, static typing) than an actual language implementation, so it is completely normal that there are many languages that have the Lisp like property, just like many languages have garbage collection.
