Why Linear Differential Equations are First Order

First, a note on format. Dⁿ(y)_x is the n-th differential of y with respect to x. 
I've used this format to keep it somewhat close to the format used by sagemath. However, I have broken down here and there and used y` to represent the first differential of y w.r.t. x.

It's a well-known fact that linear differential equations of the form:

Dⁿ(y)_x+ ... + a D(y)_x + by =f(x)

are particularly easy to solve.

Since:

Dⁿ(y+z)_x = Dⁿ(y)_x + Dⁿ(z)_x 

it is easy to split the problem into several subproblems and add them all at the end.

But matrices allow us to rearrange linear differential equations of any order into a simple first order differential equation of the form:

D(y)_x = ky