R (1976)[1] and Matlab (1984)[2] both predate Python (1991)[3], so I guess the answer is because the 'existing' language didn't exist.

As far as Julia goes it aims to bring strong typing and very fast native performance for numerical operations. Neither of which R, Matlab, or Python provide. Seems perfectly reasonable to me.

[1]: http://en.m.wikipedia.org/wiki/S_(programming_language)

[2]: http://en.m.wikipedia.org/wiki/MATLAB

[3]: http://en.m.wikipedia.org/wiki/Python_(programming_language)