SFBD Frame question

I didn't want to hi-jack another guys thread but I have always been told/taught that when you build a frame using 2X3 (or whatever your choice may be) you put it on the side where 3" is the height not the width..

I am not sure as to why? With all the crossmembers, supports and mounts I figured that it should be strong enough, No?

I have never really questioned it till now...

You run the rails tall because they are less prone to bow in the center when there is a longer piece. If you are laying it down so it is 2" tall you need to run two rails under the cab as a minimum to keep the strength up.

Most of your flex is vertical on a frame and 2x3 is stronger standing up. You can use a thicker wall 2x3 and lay it on it's side, but it will add alot of weight before it's the same strength as the thinner vertical piece.