Why are the people of Maycomb County so poor?
It is important to recognize that the book is set in the 1930s during the Great Depression. The entire country was facing economic hardship, so Maycomb was not unique its struggles.
That said, we also know that Maycomb is a small, rural town. The town doesn't create much opportunity to earn money; they are not a seat of industry or agriculture, for example. Geographically, it is far from most everything, too. So Maycomb is not exactly a tourist or economic hub. The town members appear to be scraping by and supporting themselves as best they can.
We see many of the townsfolk, like the Cunninghams, paying for services with food rather than with money simply because they are able to grow their food but unable to earn much or any cash to spend.