I don't have any experience of the bike industry, but I have spent plenty of time looking at materials substitution across many industries using structural metals. It's actually quite a complex discussion, and the trade off's are likely to be between manufacturing aspects and bike performance..
But if you can get a handle on the engineering aspects of your steels, titanium alloys and aluminium alloys, and decide which are the msot important for your application/market/cost base, such as (and this is not an exclusive list):
Material grade
Density
Tensile modulus
Yield strength
Beam bending
Effect of change of section modulus on Specific stiffness
Fatigue
Corrosion
Available finish
Fracture toughness
Heat treatment
Weldability
Effects of welding on yield stress and fatigue
Post weld heat treatment
Machinability
Formability
Cost and availability of raw materials
Cost of manufacturing process (material and grade chosen will have a large effect on this.
Experience/training of manufacturer/workforce
and finally
FASHION.
Then you will be able pronouncements on which is better.
I have no doubt that you could easily make a steel FS frame as good as an aluminium one but I would wager that the major reasons why aluminium is more prevalent are:
1) Fashion/marketing
2) Fashions knock on effect in the industry where most production outfits are geared up for aluminium manufacturing, so to make a production run of quality, competitive steel FS bike will be more expensive as the availability of mass production machines/workers/facilities is less..
Oh and this
brant - Member
The main point of titanium FS bikes, as far as I can tell, is that they are made out of titanium. This is a property that no other material has. No other material other than Titanium lets you make a titanium bike.
Steel is similar to this in many ways.