It is touted as largest cash robbery committed on American soil at the time then why Goodfellas movie didn't show how Lufthansa heist on screen?
Because it's not necessary.
It's clear from Henry's reaction in the shower that he knows that Jimmy was responsible for the heist and all we need to know is that it was successful.
We had previously seen Jimmy use his connections at the airport to carry out successful robberies (Air France) so we don't really need to see the heist take place.
HOW is not important to the story being told here. What is important is the results of the heist and it's impact on the participants.
Jimmy organizes a crew to raid the Lufthansa vault at the JFK Airport, stealing several millions in cash and jewelry. After some members purchase expensive items against Jimmy's orders and the getaway truck is found by police, he has most of the crew murdered. In his voiceover narration, as dead bodies are being discovered all over the city, Henry theorizes that Jimmy would have killed them anyway rather than share the profits of the heist.