Why was Tutankhamun famous?

Because his tomb is the only tomb of a Pharaoh discovered in the Valley of the Kings that had not been robbed. All of the treasures buried with the king were still there.

Most Pharaonic tombs had been looted over the centuries, so only the tomb itself and none of its actual contents remained. Not Tutankhamun though, his tomb was crammed full of “wonderful things” in Howard Carter's own words.

As a matter of fact, Tutankhamun renown in the modern world is somewhat ironic, given that during his reign he actually accomplished fairly little of note. Today, he is, along with Cleopatra and possibly Ramesses the Great, among the only Egyptian Kings that most ordinary people can name.

It is unlikely, however, that most ordinary people living in Ancient Egypt some time after Tutankhamun death would be able to recall him, or at least name any of his achievements, which were few.

Given that he was a child for most of his reign and died at 19, with the proceedings of the Royal Court likely being overseen by the vizier Ay, Tutankhamun was probably regarded as a footnote in history, insofar as a monarch viewed as a living god could be considered inconsequential.

It seems that even soon after his death, the Egyptians didn't waste too much time mourning, despite the riches interned within, the tomb of Tutankhamun is uncharacteristically small for king, and the priceless treasures buried with the Pharaoh were literally stacked unceremoniously on top of each other to maximise space, rather like one would cram bicycles into the back of car.