Here are also some interesting replies.

Problem: going to space is so costly, it incites clever engineers to find other far cheaper ways to achieve the same goal. Platinum so badly needed? Well, find another substance on Earth that does it more cheaply!
And thus, space remains prohibitively expensive !
Space is neither the solution to expanding population: there is still plenty of room and the wealthier you get, the less children there are.

The market is not ready for manned space. Far from it!

Concerning asteroid material: it is extremely expensive to get to an asteroid and we are nowhere near able to extract specific raw material from asteroids (there are no mineral ores in space); these problems add to each other.

The oil industry reached very cheap results thanks to years of use, huge markets and huge dependancy.

The shuttle was a genuine attempt to reduce the costs, but it failed. Luckily (or unluckily), it still had a big enough political appeal to last 25 years (almost 30 years, btw).
But it didn't make space more affordable.