South Australian here. Lots of reasons.

1. Politics

Many of the later reasons stem from this reason. Unfortunately.

The South Australian state government is Labor (leftish), whilst the Australian Federal Government is Liberal (rightish). The Federal Government have been very supportive of coal and talking down renewables aggressively and, sometimes, untruthfully. So I'm not sure whether SA Labor is actually supportive of renewables for renewables / environmental reasons or just as a political point-of-difference, but the end result is pushing renewables pretty hard. I think SA was over 50% powered by renewables in 2017.

References:
http://www.abc.net.au/news/2017-03-16/jay-weatherill-tirade-...

2. A large coal-powered generator was recently closed down in Port Augusta.

This increased South Australia's dependence upon renewables (solar and wind), gas, and the interconnector from Victoria (neighbouring state).

There was a lot of discussion around this about how it's too big a risk and the State Government needs to step in to prevent its closure. It was revealed that the owner of the plant offered to sell the whole shebang (plant, land, the works) to the South Australian State Government for $1.

Personally, if someone offered it to me for $1 I'd be scared of accidentally hanging myself on all the attached strings. Gives you an idea of what it's worth...

It was shut-down as it could no longer compete with power prices.

Refer:
https://en.wikipedia.org/wiki/Northern_Power_Station_(South_...

3. South Australia had a large system black incident towards the end of 2016.

This was caused by a massive storm that brought down 13 (I think it was 13) pylons carrying the high tension wires feeding various parts of the state.

The conservative media blamed this incident squarely on renewables and the shutting down of the Port Augusta power station. The reality is far more nuanced, including the fact that the Australian Energy Market Operator (AEMO) didn't know about cut-off settings that some of the wind farms had configured. Were these settings different, the wind farms would have actually (likely?) managed to ride the state through the system black event.

These settings have since been updated and standardised, apparently.

4. South Australia relies on the interconnector from Victoria for varying degrees of its power.

I believe they're aiming to minimise reliance on this, and have more local power generation, for which solar and wind are ideal, and storage goes hand-in-hand with that.

5. South Australia has a few gas power stations, but gas is more expensive than coal-powered stations.

Australia did an international deal with Japan for gas trading a few years ago. The way things have turned out, it's cheaper to buy Australian gas from Japan and ship it back here, than to buy it locally. That situation may have been resolved in the last 12 months though.

Gas generators have also been accused of gaming the energy market by shutting down, or refusing to start-up additional units, thus reducing the supply and increasing the spot price.

6. South Australia has some of the most expensive electricity prices in the world. This means there are lots of opportunities for political points scoring.

Being seen 'doing something' is of value, whether it's effective or not. Depends on the spin.

See Gold-plating electricity network:
http://www.abc.net.au/news/2017-07-18/australian-gold-plated...

My personal experience with, and research into, increasing power prices:
http://electricity.atcf.com.au/cost-component-percentage-inc...