About any lumber will do..... it mainly depends on the person building the stand and proper selection of wood.
Using 2x4 is easy to use and lets be honest, most that end up building a stand have poor to introductory wood working skills, limited to no real tools, etc.. at their disposal. Slapping a few 2x4 together can be done by anyone so ends up being the material of choice by most DIY aquarium builds plus easy to find plans and cut list so it takes almost any error out of building. The garf calculator and plans are about fool proof.
I used 4x4 in the last two stands I built but had to do more with what I had on hand or for the looks (as mentioned overkill) and with proper selection of wood no more concerns with warping than with any other wood.