There's a few reasons some people prefer steel. 1) You can weld it pretty easily, if your frame cracks. I guess this is nice if you're bikepacking through Nepal or something, but I don't really see the practicality for normal people. 2) It's cheap (well, not any more). 3) It's more flexible than aluminum, so some people prefer the way it rides.

These days, I'd say #3 is the only really valid reason for most people. Steel bikes are generally heavier than aluminum however, but it is possible to get them close if you use a high-grade cromoly steel, but then that removes most of the cost advantage.

An interesting feature of steel frames is demonstrated by Surly frames: their rear end spacing is currently 145mm, which is neither 142 nor 148 (the standards). What they say is that because it is steel, you can bend it a couple of millimeters without any negative side-effects, the frame will tolerate it and will not break. So your frame can accommodate both sizes.

From university I learned that aluminium and steel both have around the same weight to strength ratio. The fact they use aluminum for airplanes is that steel will become too thin and is therefor not stiff enough.

I think for bikes the opposite matters, with aluminium at required strength you will get a very stiff frame.

Durability is part of it, but mainly I prefer the way it rides.