There is a specific reason that singular homology with simplices is simpler than singular homology with cubes.  You would like the homology of a point to be "trivial" according to the Eilenberg-Steenrod axioms. (That is, $H^0 = \mathbb{Z}$ and the others are trivial.)  However, if you look carefully at the chain complex of maps from cubes to a point, it is not true.  The homology groups have to be corrected by hand by annihilating certain degenerate chains.  If instead you carefully use the standard ordered simplex --- not just an abstract simplex floating in space but one with numbered vertices --- then it works automatically.  On the other hand, products of chains look a bit simpler at first with cubes.  They are only barely simpler though, because of the magic of simplicial sets.  So on balance simplices are nicer than cubes.

Likewise, at the entirely rigorous level, defining singular homology with "disks" is not really a complete proposal, and not necessarily a simple proposal if you were to flesh it out.  Do you mean maps from "all" balls, or some specific collection of balls, or just one standard ball?  What is the boundary operator?

You can think of singular homology as a conversion from topology to combinatorics, attained by building a simplicial set (which is a generalization of a simplicial complex) from a topological space X, and then taking the simplicial homology of the combinatorial object.  There is a specific way to do that with simplices, and an analogue with cubes that gives you a cubical complex.  For instance, in the case of simplices, even if you start with a point, you get an infinite-dimensional complex with one simplex in each dimension.  Presumably you have in mind bulding some CW complex from a topological space X, and then taking its CW homology.  But what do you really want to connect to what?

It is also true that once you have a definition of homology, you can use a mapped-in disk to define a cycle or a relative cycle, which is then unique up to homology.  But that is not the same as defining homology.