Justice for Nirbhaya: Why it took so long for our judicial system to punish the rapists?

Even as a Madhya Pradesh court fast tracked its landmark judgement and gave death penalty to the man convicted of raping a 9-year-old girl in may this year, Justice continues to evade Nirbahaya. It's been six years since Nirbahaya was rape and brutally killed by her six perpetrators. But none of them have been executed for their heinous crime. While one among killed himself in jail premises, the juvenile was acquitted after serving three years in jail. The other three have filed review pleas in the Supreme Court and will be heard by the three judge bench led by CJI Dipak Misra today. The bench will give its final verdict in the case. But the question is why has it taken so long for our judicial system to punish the rapists of Nirbhaya? Isn't justice delayed is justice denied?