I'd love to see a metric like average revenue per user (ARPU) that might give some nuance to the public attendance numbers. We already know that our TV revenue is among the lowest, but my guess is that the revenue per fan or ticket sale is pretty low as well.
My guess is that the biggest factor is that Milwaukee is well below other small markets like KC or MSP in measures like GDP https://www.statista.com/statistics/248083/real-gross-domestic-product-gdp-of-the-united-states-by-metropolitan-area/, there's simply less money around to spend, both by individuals and businesses.
Fans of the Marlins, Mariners, and Twins have more of a right to be outraged, owners sitting on goldmines who are either pocketing the profits or too incompetent to convert the opportunity to revenue. Twins in particular, who had a history of success, a loyal fanbase, yet had a billionaire owner threatening to have the team contracted to get a free stadium.