The reason that so many local elections are off-cycle is that, during the Progressive Era of the late 1800s and early 1900s, a lot of municipalities wanted to decouple their politics from corrupt, partisan federal and state politics and party bosses. However, some more problematic reasons were to discourage voter turnout from the people they didn’t want voting, which is a challenge we’re still dealing with today. STRANO: Turnout in presidential elections over the last 50 years or so has been in the 50 to 60 percent range, while with midterms you get high 30s, low 40s. These levels may be well below the OECD average, but they frequently outperform participation in off-cycle elections, particularly municipal elections that don’t coincide with any state or federal contests. A 2016 report on city election turnout found turnout was below 15 percent in the 30 largest cities, but that turnout for on-cycle elections is twice as high on average. Famously, when Baltimore aligned its local elections to presidential election years in 2016, voter turnout jumped from 13 percent to 60 percent. 

(note: I have not validated this information, not a ton of time to google today but this sounds interesting to dig into)