Guitar: Because Yui thought it would be easy
Piano: Because Mugi knew a little about it, and thought Mio and Ritsu were cute
Bass: Because Mio was too embarrassed to play guitar and the only reason she's doing this is because Ritsu badgered her into being in the light music club in the first place.
Drums: Because Ritsu wants to play the fucking drums.
Motivation: Only one member has it.
>Because Mugi knew a little about it
Hadn't she been playing since she was about 4 or so?
I thought she wanted drums because her fingers are too clumsy for fine-motor movements required for keyboard/guitar/bass?
Yes. But remember originally she was looking to join the chorus club. In fact had she not made the mistake of going to the wrong club the whole thing would have never happened.
She didn't know they were hard until she tried them in the show. She never even tried anything else before.
I think Mugi sniffed out the lesbian musk Ritsu and Mio were giving off and found her way to the Keionbu.
Mio is the group's backbone on and off-stage. That's why she plays bass.
Pic probably not related to the post, but still nice.
Describes the show dynamic pretty well.
I don't trust Mio. She's clearly sinister.
Ritsu didn't speed up her parts.
She was like a metronome.
Describes K-ON pretty well
What about Azu-nyan? She was the most devoted member of the club and diligently practiced.
The extra second guitar no one really needs but adds a little spice to things.