Have we tried? State aid rules could theoretically block us from doing so, but their track record suggests that it’s unlikely they’d do so. Without even mentioning that we were fundamental to blocking attempts to stop the flow of cheap Chinese steel into the EU in the first place. Sigh.
The two main exceptions to state aid in industries are for defence purposes or where failure of an industry/company would create a “serious disturbance in the economy”, hence why the banks were bailed out in 08/09. Unfortunately the collapse of British Steel isn’t supported as another steel provider (probably China) will just pick up the slack - ie no impact to the wider economy.
I don’t know about your point about not stopping cheap steel coming in from China but it wouldn’t surprise me based on recent governments. But that doesn’t mean it should be literally impossible to provide state aid to save 24,000 jobs.