@halirutan has already answered the OP's actual question, but I think some discussion is warranted about whether this question fits this site.
Normally questions here are expected to stem from an actual problem you are having, and they should be objectively answerable.
This question is very much a borderline case. I did not vote to close it because there was already a reasonable answer when I saw the question and because I did notice when the CS moderators were asking whether migration would be okay. Without these two factors I may easily have voted to close as well.
On the surface, the question really doesn't belong. It is asking why the designer of Mathematica chose a specific design. It doesn't give a context and it doesn't make references to any specific applications of Mathematica (e.g. what are the relative merits of term-rewriting when applied to computer algebra?). Even if it did, there are computer algebra systems using very different paradigms (Maple, Maxima, Axiom), so the answer would easily contain many subjective arguments. (It is interesting to note that both Wolfram and Maplesoft market their choice of programming paradigm as an advantage over the other!)
On the surface, the question reads like: What was the designer of Mathematica thinking? I'm sure many interpreted it this way (see the comments under the question) and this is obviously not a question that can be answered by anyone else than the designer himself. Indeed, the answer links a talk by Stephen Wolfram where he talks about the creation of Mathematica.
I think the only way this question makes sense is if we interpret it as being about the history of Mathematica. If we view it from this angle, it suddenly makes sense. It is not a question which stems from a practical problem, but arguably it does fit the StackEchange format in general. There is a History StackExchange after all.
There are no precedents for such questions (that I am aware of). But perhaps we can have a discussion whether such questions are welcome. The history tag is so far about things like
Personally I don't mind allowing history questions, as I don't expect that it will get out of hand. But I am not sure if most such questions would get good answers.