About 1.8M potential voters live on Houston soil yet around only 200k participated in the mayoral election.
There are two issues with this.
One is that VAP =/= voters. Measuring by CVAP puts the number 400K smaller, and that's before considering everything that would prevent someone from registering, both personal and legal. I'm not sure if Texas breaks things down by city, but Harris in 2020 had about 90% of the CVAP at that time on their registered voters list.
Then theres the fact that any election not on the regular date in November, and not in a even year is going to have horrible turnout. Which is why in some states there is increasing pressure to standardize local contests.
The argument for having local elections in off-years is that they don't get overshadowed by other races. People aren't going to pay close attention to a mayoral race the same year as a presidential one, or even a gubernatorial one if held in the midterms (also why most states hold their gubernatorial elections on midterm years, although not all of course.)