The #1 reason here is one of the most important. You have to be able to draw your landing ellipse in a spot that you can be confident the rover can safely land. Technological advances have shrunk the size of this ellipse considerably, check this image from JPL for comparison of past missions:

https://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=439...

Another reason is the decisions are made ... ok, perhaps only influenced by committee ... you have to satisfy at least a significant subset of the Mars research community with your site selection. See this site for public information on the process:

https://marsnext.jpl.nasa.gov/

There are a number of other hard engineering concerns, like limits on latitude based on heating (or solar power) requirements, e.g. if you go to far north or south you have to use all your available power to heat the spacecraft and won't have any left to do science. I don't remember the exact values, but I think existing technology puts you in a +/- 60 degrees from the equator. I think it was even less than that, but I don't recall exactly.