I find this a very odd question to be asked on this list. To me, one of the main attractions of the "everything" thesis is that it provides the only possible answer to this question. Viz: as Jonathan pointed out, mathematical objects are logical necessities, and the thesis (at least in Tegmark's formulation) is that physical existence is identical to mathematical existance.
Despite this attractive feature, I'm fairly sure the thesis is wrong (so that there is just no answer to the big WHY?), but that's another story.
====================================================== Dr J. P. Leahy, University of Manchester, Jodrell Bank Observatory, School of Physics & Astronomy, Macclesfield, Cheshire SK11 9DL, UK Tel - +44 1477 572636, Fax - +44 1477 571618