Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower? In my textbook, the H–R diagram’s y-axis is $\log(L/L_{\odot})$ and values are higher as we go higher on the scale, but the x-axis is $\log(T_\text{eff})$ and gets smaller as we proceed to the right.

This confuses me. Why not let the x-axis go from lower values to higher values? This will also help visualising the “linear” relation between the luminosity and the temperature of a star in the main sequence.
Originally, what was plotted was luminosity against colour, and by colour I mean the wavelength of the peak intensity. Short wavelengths on the left, and long on the right, as you would expect. 



Now since stars emit (nearly) black body radiation, there is a close relationship between colour and temperature. I suspect that the reason that the x axis isn't inverted when temperature is used is just inertia:- We have always plotted out H-Z diagrams like this and we will keep doing so.