Because Genius was popular enough it would piss off users if their results didn't show up on the first page.
Clearly Google seems to agree that Genius has the best results based on the efforts they've taken to steal and even hide the fact that they're stealing from Genius.
Just to be clear, Genius may be small compared to Google, but they are the de facto source of lyrics for music. Can you imagine if users suddenly no longer found Google useful for figuring out lyrics? It would genuinely cost them users switching to Bing, DuckDuckGo, etcetera.
Because the VC money, the board is tied. In the end that's the way with all the SV startups that gives them insane advantage. Too many vested interests.