Skylab: Why don't we live in space yet?

Commander Charles 'Pete' Conrad undergoes a dental examination in zero gravity on Skylab

The first US space station, Skylab, was launched forty years ago with a simple but far-reaching brief: expand man's knowledge of the Sun and prove that humans can live and work in space for extended periods. Three separate crews successfully achieved that, so why aren't more of us living in space yet?

On May 14th 1973 NASA launched the Skylab space station into orbit. After a decade defined by lunar exploration and the Apollo programme, space travel was moving into a new age, one of space stations.
"Skylab will represent a milestone of paramount importance in the American space programme," wrote Dr. Ernst Stuhlinger, Associate Director of Science at the Marshall Space Flight Center, and Leland F. Belew, Manager of the Skylab programme, in 1973.

Ambitious project
Weighing 77.5 tonnes, Skylab was the largest craft yet to be launched into space. It needed to be, to house the well-equipped laboratories and living quarters for three astronauts, suitable for extended periods of time.
David S. Akens, a member of NASA's historical staff, stated at the time that "Skylab is the most ambitious project in space to date".
11 days later, the first crew left Earth, heading for Skylab. They were led by Commander Charles 'Pete' Conrad, a veteran of three previous space missions.
The crew immediately had difficulties to contend with. During the space station's lift-off, a crucial meteoroid shield had been ripped off, along with a solar panel. Temperatures inside the Skylab workshop, which was facing the Sun, had reached a sweltering 52°C.
The crew managed to deploy a parasol sunshade, which lowered temperatures to 24°C. By June 4th, 10 days after they had arrived, the workshop was fully operational.
Collectively, three separate Skylab crews spent 171 days in space - each new crew breaking the previous crew's spaceflight duration record.
It was a remarkable achievement. NASA had begun operations less than 15 years previously, and the first rudimentary designs of Skylab had been sketched out in 1966.

Skylab concept drawing from August 19th 1966

Integrated Space Plan Optimism at NASA continued well into the 1980s, with the launch of the space shuttle. Designed to be reusable, the shuttle could transport crew to any orbiting space station and back again.
In 1989, a US aerospace company, Rockwell International, mapped out where all this would lead. The Rockwell Integrated Space Plan - an immensely detailed vision of humanity's future in space - began with the shuttle programme, and outlined the next 120 years of human space flight.
According to the Integrated Space Plan, an International Lunar Base would be established by 2009. By 2029, mankind was expected to have engineered an operational Mars base.
And around 2100, large-scale human expansion into the cosmos would begin.