In 1971 the first aluminum tanks showed up in the US. They were introduced by US Divers. They were Special Permit tanks with 72 cu ft capacity and were stamped 2475 psi. They were 6.9 inches in diameter (the same as a steel 72), but much taller. They were 9 lb positive when empty (they were called floaters, buoy, etc.)
It was also advertised as not having a need for getting it + stamped in the future.
Most divers with these buoyant tanks would add a steel 72 boot (rounded bottom tank boot on a square bottom tank) and used the space inside the boot to add some lead. It made the tank that much longer (and heavier), but at least the buoyancy was manageable.
The original aluminum tanks were awful, but they were advertised as rust proof (corrosion resistance). Corrosion on steel tanks was a big issue. Yearly internal inspections were introduced for the same reason.
I almost bought one of those aluminum tanks as my first tank. I am so glad I didnt. They were advertised as a great improvement and with a great introductory price (the steel tanks was still less expensive).
The original aluminum 72 was soon replaced with 3000 psi tanks. In particular the US Divers aluminum 80 Professional caught on quickly for its extra air. The newer 3000 psi tanks were not as buoyant
they were actually a big improvement.
Again the corrosion resistance was the original big seller. Aluminum tanks were originally more expensive than steel 72.
In the Caribbean were large fleets of rental tanks get a lot of abuse with minimal internal tank inspection the aluminum 80 does make sense, especially since it is less expensive for the dive operators.
Added:
I love my steel 72 (I have about 12 of them) and my other steel tanks, but my favorite tank is a rental aluminum 80 down in the Caribbean.