Because geniuses tend to be particularly averse to labelling themselves as something that has connotations of extreme superiority.
It's the reverse Dunning-Kruger effect - idiots are less aware of their limitations and thus are more likely to be baselessly confident. Legitimate geniuses are the most aware of their limitations of anyone, and thus are less likely to trumpet the 'genius' title.
Why would you expect otherwise for other geniuses?