https://en.wikipedia.org/wiki/Sixth_Amendment_to_the_United_...
TL;DR the Sixth's right to jury trial doesn't apply for criminal charges carrying less than sixth months maximum sentence—even if there are multiple such charges that may add up to more than that.
Gonna go out on a limb and guess that plenty of US jurisdictions don't voluntarily pay extra money to make those jury trials if they aren't forced to.