Why would a song be considered to be written in F# when no other chords in the song are in that key?

Example, White Rabbit by Jefferson Airplane. F# G F# G A C D A is the chord progression. The guitar solo is out of a F# minor scale more or less. That scale contains at least F# A and D.

It is pretty obvious what key something is in if it mostly stays in key. It is not so obvious when most of the notes are out of key.

Is this just a question of ease of writing in standard notation or first notes defines the key?

Thanks,

Billy
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Sears, Roebuck and Co guitar, Black Diamond Strings, RCA Victor Radio