Why am i not able to get a legendary item? [duplicate]

The drops are totally random and its on your luck. Yes Its possible to get legendary drops on any difficulty settings. The lowest level on which you can receive the legendary drop is 54, which you have achieved already. Source

However if you are really desperate and want to increase the chances of getting the drop then you can try Mods which will increase the drop rate. You can even customize those drop rates and set it to your liking to get more drops.

Here is a Mod for increasing drop chance. Increase Drop Rates Mod