The short answer is: the diatonic scale in a given key consists of notes whose frequencies are those of the keynote times certain simple rational numbers; the white notes on the piano are those that belong to the diatonic scale based on C.

What's special about those simple rational-number ratios? Answer: on most musical instruments, notes whose frequencies are in simple rational ratios sound nice together. This turns out (surprisingly, at least to me) to be a fact about the instrument and not merely about the notes; when you play a given note on a given instrument, you get (kinda) sine waves whose frequencies are those of the given note, plus some higher frequencies; exactly what the higher frequencies are and how much of each you get depends on the instrument. For most instruments, the higher frequencies are integer multiples of the fundamental frequency, and that turns out to mean that the good-sounding combinations of notes are ones with simple rational frequency ratios; but there are instruments that behave differently (e.g., a tuned circular drum; or you can make a synthesized instrument that does anything you like) and different chords will sound good on them.

For much more on this, see William Sethares's book "Tuning, Timbre, Spectrum, Scale" and his web pages at http://eceserv0.ece.wisc.edu/~sethares/ttss.html where you can find, e.g., some music in unorthodox scales performed on (synthetic) instruments designed to make the harmony sound good.

For instance: listen to http://eceserv0.ece.wisc.edu/~sethares/mp3s/tenfingersX.mp3 and hear how out-of-tune it sounds. Now try http://eceserv0.ece.wisc.edu/~sethares/mp3s/Ten_Fingers.mp3 which has exactly the same notes but played on a synthetic instrument designed to make the harmonies work.