Well, I dive mostly cold water and mostly aluminums, so here is my opinion on that:
I think the situation translates well from Germany to the rest of Europe:
Hollis recommended it when they started the first sidemount classes here.
Afterwards it has been repeated so often, that when people ask they always get the impression that 'everyone' thinks aluminum is best for sidemount generally.
It is also the typical recomendation of people comming back from the cenotes with only basic knowledge from seeing sidemount divers.
Another reason this recomendation still keeps being repeated is that most vacation locations have aluminums, so many people try to prepare at home with similar types.
You see them in most videos (since those come mainly from warm and salt water areas) and people think it is part of the concept or an explanation for the sometimes unbelievable ease shown in the videos.
There are also a lot of wrong ideas influencing people:
- People still tell you in Germany that aluminums need to be certified less often, even though that changed in 2013 or something.
- Many think aluminums weight less on land
- Many people fear rust (even with compressors here always being in good condition nowadays)
...
White paint is not attractive. Especially in Germany many think the tanks have to be painted white, according to some unknown law or something.
And it can be quite hard to find anyone to offer bargain prices for painting them another color, airbrush often would cost the same (meaning: cost to much).
Many like the dirty beast or mirror-finish design and both can be easily personalized with sanding paper.
Sidemount systems cost a lot of money. People affording something like that tent to try to get perfect sets of additional equipment.
Aluminums are 'attractive'.
There is also the convenience aspect of aluminums standing without a tank boot - much more comfortable with filling stations prepared only for backmount.
My personal reason is that it is a lot easier to have tanks that can be used as stages or sidemount tanks according to situation.
4 tanks is the ideal number to have at home. 4x 80cft stages can be used in any combination from one to four tanks and from one to four dives.
With steel tanks you can now add an extra pair of extra light tanks (owned 8liter 200bar - best sidemount tank ever) and a pair of extra heavy ones (I have a pair of 19liters).
Still, the aluminums will always be my 'allround tanks', the steels are for 'special occasions' and there is no dive I could not replace them with aluminums for an only slightly less optimal solution.