Wikipedia has some notes on why least squares, and how you might get there from other assumptions: https://en.wikipedia.org/wiki/Least_squares#Statistical_test... .

Also, quadratics are just much easier to work with in a lot of ways than higher powers. Like you said, even powers have the advantage over odd powers of not needing any sort of absolute value, but quartic equations of any kind are much harder to work with than quadratics. A local optimum on a quartic isn't necessarily a global optimum, you lose the solvability advantages of having linear derivatives, et cetera.