It's the most obvious thing in the history of obviousness.

We know for a fact that Luffy doesn't give a shit about what other people might think of his Nakama.

And its not like the Water 7 arc where there's alot of shipwrights and you don't know which one its gonna be(and then ends up being the dismantler). Brook is a musician. He's THE musician. The Musician of Destiny!!