• "Gather 'round the piano while I play something" usually sounds either presumptuous ("so I can impress you") or hokey ("for an old-fashioned sing-along"), and just sitting down and playing tends to have a pretty dramatic and immediate effect upon the "room", while noodling on a guitar is far less imposing and allows a crowd to form more naturally.
• Limited availability of ivories to tickle - pianos are hard to lug along on camping trips or midnight runs to the beach
• The piano-free inclination of most of today's Top 40 and pop music (this was less of a problem when Billy Joel was still cranking out great albums; e.g. Storm Front)
• Chicks prefer guitarists (see women who like jerk guys); I've been called a "piano hero" twice in my life, compared with my best friend Will who has been called a guitar hero more times than either of us can count. (Note that he is the exception to the insinuation earlier in this bullet item.)

See also on the selection of musical instruments.

I was watching VH-1, or MTV or something and the guitarist for a band (no, I don't remember which one) said that there is only one reason anyone picks up a guitar for the first time, to get laid. Oh, sure, they learn to love the music, but when they first decide to pick up the guitar sex is the only thing on their mind.

As an ex-aspiring guitar player I agree.