A little late to the party on this post but I work in this industry so I wanted to reply since it doesn't seem like anyone has given you a good answer. I've worked on many studies in site selection for pharma companies and I've worked to support clinical research sites in their advertising campaigns. For some background, to run a study in a town like yours, there has to be a physician willing to oversee the study out of a doctor's office. Usually these are physicians who are interested in conducting research studies and who've worked with pharma companies in the past. So when picking where to do studies, the pharma companies tend to go to the doctors that they're familiar with and that they know can successfully recruit patients and run a clinical trial.

So first off, there are studies going on at every city in America, so your town isn't particularly unique in that it was a place selected for trials. What happens is that sometimes doctors who run studies realize how much money can be made so they decide to open up a research clinic. These clinics employ staff who do nothing but see patients for studies all day, running many, many clinical trials. I've sometimes heard these practices referred to as "research mills" in the industry and pharma companies tend to be wary of them. Their staff is usually overworked, they have high staff turnover, and their work is often sloppy, which is not what you want when testing investigational drugs. Usually these clinics run studies for rather common "lifestyle" diseases like diabetes, heart disease, or fatty liver. (More complex studies such as oncology studies are typically ran by cancer clinics that specialize in various types of cancer treatment.) These lifestyle disease studies are fairly simple to execute and usually just mean taking vitals, collecting some blood samples, and sending the patient on their way with a refill of the experimental drug. It's likely that the advertisements you're seeing are from a clinic that's carpet bombing advertising to try to appeal to members of your community affected by these types of diseases but may not be able to afford regular healthcare. Or it could be that they're trying to appeal to patients who have these common diseases and would like to make a bit of money, as patients often receive $75-100 every time they have to come in for a study visit, which is often 10-20 visits over the course of a couple of years. I would guess that you would have seen similar ads if you spent time in lower income areas in your former city as they are being conducted in bigger cities too. For statistical purposes, studies need high diversity in many different areas, with a good mix of different races, income backgrounds, and also from both rural and urban areas, among other factors.

If you want further insights on your question, I'd recommend reposting it to r/clinicalresearch.

If you're interested in looking up your town or city to see what studies are going on near you, I would recommend checking out ClinicalTrials.gov, it gives a listing of very clinical research study that's going on in the US.