The reason the Tories don't want an inquiry is because it will show just how awful Thatcher really was.
Couldn't put it better, BowieFan. Those who still idolize Thatcher will go to great lengths to preserve her image. An enquiry examining her personal determination to teach the miners a lesson, which lead to military tactics, horses and dogs being used against members of the public, would reveal corruption even diehard fans would find hard to ignore.
What happened at Orgreave was shameful, and what happened after was at least as bad. 95 miners were charged with offences including riot, which carries a life sentence. But as the police evidence was produced it became clear that there was extensive perjury, with statements of individual officers identical. Police collusion was blatant and the trial collapsed. All credit to the judge. Later damages were paid to some of the defendants on the grounds, among others, of malicious prosecution.
Human rights lawyers say the issues around police conduct at Orgreave resonate to this day, and insist an enquiry is essential to examine policy at the time, and since. What happened at Hillsborough would not have happened without the corrupting effect of the use of police and subsequent perjury during the miners' strike.
Gosh, that's a screed! It's not like I'm passionate about it or anything. 
TLDR: The police were used as a political tool at Orgreave, and 100s were ordered to commit perjury. Its legacy of contempt for civilians created the "us and them" climate that resulted in the events at Hillsborough.