Hi Jan, I believe that there were two problems with early aluminum that made it a challenging material for the use of bicycle frames.
First, the very early aluminum alloys (and I believe all aluminum is an alloy of some type or another) had strength deficiencies and tended to be crumbly when broken. A lot of development time was needed to make stronger aluminum with different properties, and to determine the recipes for various results. I have worked with a lot of pre-war aluminum and it is very different than what we can buy today. This can be seen in its consistency, how it work hardens and gets quite brittle and how it breaks when pushed to its limit. That is why there was so much concurrent exploration of magnesium alloys. At the time many engineers saw these lightweight materials as alternatives to cast iron rather than to steel.
And this leads to the second problem. There were difficulties in joining aluminum that were not solved well in the earliest decades of the twentieth century. In bicycle frame, Barra seems to be the first one (that I know of) to come up with a practical weld, but that depended upon the availability of new"weldable" versions of the aluminum tubing--that is to say improving metallurgy. I think a lot of this development work was spurred on by military applications, especially in the 1930's.
I think when we look back on all this we have to remember that use of these "advanced" materials was a moving target and that developments (and availability of materials) did not spread throughout the cycling industry evenly.
David Cooper
Chicago, IL