Chris Mulligan emailed Ken Thompson to find out. Chris initially assumed it was because * looked like a point and & was beside it on the keyboard.
From: Ken Thompson < ken@google.com >
near on the keyboard: no.
c copied from b so & and * are same there.
b got * from earlier languages - some assembly, bcpl and i think pl/1.
i think that i used & because the name (ampersand) sounds like "address." b was designed to be run with a teletype model 33 teletype. (5 bit baud-o code)
so the use of symbols was restricted.