In general, alloy design is an extremely high dimensional problem. You have composition, the forming processes, and heat treatments which all can profoundly affect the properties of the finished product. It's only since the fifties or so that we've really started to understand the internal structures of metals and how they affect properties like strength, and there are many open questions yet.
[1] http://link.springer.com/article/10.1007%2Fs11661-008-9593-3 [2] https://en.m.wikipedia.org/wiki/Precipitation_hardening
We probably spent as much time studying ferrous alloys as we did on Aluminium, Titanium, Copper and Nickel combined.
That said I suspect the reason is likely patent related. Metallurgy is not a new field but Aluminium alloying is relatively new wasn't really until after WW2. Alot of industrial research tends to be locked away behind patents.
General hardening is no problem. The challenge is when a company needs a very specific hardness. There is a fair amount of trial and error with the heat treater (at least at first) before they get it right.