I found this on the interweb -
"In the opinion of Jeff Pressing, a psychologist who has studied improvisation extensively, vocal improvisers lack the benefit of "feedback redundancy" that instrumental improvisers have. All improvisers use feedback from their playing in order to judge what to play next; the more feedback that exists, the easier the improviser's task is. For the instrumentalist, aural, visual, proprioceptive (i.e. body awareness), and touch feedback work in tandem. For the vocalist, however, only aural and proprioceptive feedback are available. Pressing uses this discrepancy to account for violin improvisation being more difficult than sax improvisation, and vocal improvisation more difficult still: "For every first-rate scat-singer in the world", he writes, "there must be 500 talented jazz saxophonists".