Like someone said above, one reason was his vocal range.
He also was very innovative. He created the rock Opera and Queen was also the first band to release a music video with Bohemian Rhapsody. He also had a large role in popularizing androgeny during that era.
They had many #1 hits, which is always a #1 (no pun intended) factor in creating a legendary band. Freddie was composing the majority of these hits. There was no Queen without Freddie; so after he died, that's why they could not continue on.
As someone above also said, they were great live and their Band Aid performance was considered the best live rock band performance ever. In addition to Queen as a band having great showmanship, Freddie in particular would steal every show. He was described in a documentary to have the demeanor/prance of a peacock on stage.
And finally and least importantly, his death probably had a factor in solidifying his place as a legend. Taken out before he was past his prime.
Mostly a documentary regarding Freddie