Violent crimes were reported about half the time for the period of 2015-2019, with slight variations from year-to-year:

I suspect more crimes will have gone unreported in 2020 (a year of strong anti-police sentiment), but those numbers aren’t in yet. As for why victims don’t report crimes, the most common one is that they dealt with the matter another way. Other reasons include: didn’t consider it important enough to report; police wouldn’t or couldn’t help anyway; fear of reprisal; and not wanting to get the offender in trouble. Here’s how the relative importance of those reasons breaks down by urban, suburban, and rural place of residence:

References:

Bureau of Justice Statistics, “Victimizations Not Reported o the Police, 2006-2010”. August 2012. Authors: Lynn Langton, BJS Statistician and Marcus Berzofsky, Christopher Krebs, and Hope Smiley-McDonald, RTI International.

Bureau of Justice Statistics, “Criminal Victimization, 2019” September 2020. Authors: Rachel E. Morgan, Ph.D., BJS Statisticians; Jennifer L. Truman, Ph.D., BJS Statisticians