Well, I've been building steel frames for over 15 years, and I still have a lot to learn - how long does it take to learn to lay up a carbon frame?
It's simple economics and economies of scale. Aluminium frames took off not because they were lighter - they often weren't - but because aluminium tubing of a given strength is much thicker, so easier to weld. Learning to braze or weld 0.5mm steel takes a long time, learning to weld 3mm aluminium is quite a bit faster. The initial costs are higher - especially once you get on to hydro forming and carbon - but once you've invested in the tooling the unit costs are much lower because the labour costs are lower.
There's a comparable example - mouldings for canoes (and recently faired recumbents). They used to be almost all laid-up fibreglass which is cheap to get started with, but time-consuming. Then when rotation moulding came along, it made a huge difference - your first canoe cost £10,000 because you had to pay for the mould, but your second cost £10.