We're also exploring Titan, Venus, the Moon, Jupiter, Saturn... the list kind of goes on.

Mars is the easiest place to get to and stay on, aside from the Moon, and is much more interesting than the Moon - thus, more interest. (Venus is harder, the gas giants and their moons are further and thus more expensive.)

(edit: I removed "Venus", because I don't think we have anything actively orbiting it, nor any specific plans to launch anything.)

(edit 2: wait, JAXA has something! https://en.wikipedia.org/wiki/Akatsuki_(spacecraft) )

Mars is close, it's not got hellish Venusian weather, it's got a clear atmosphere that's easy to look through, you can roll rovers around on the surface pretty easily. If you want to generalize your ideas about what geology does, then Mars is a nice place to observe that isn't Earth. And if we are very lucky, we will be able to generalize our ideas about life by finding some there. It also has interesting chemistry, weather, etc. It's like Earth, but different. You can test theories about how planets form against it.

It's easier to image the entirety of Mars than it is to image, say, the Pacific. It's easier to send a rover to Mars that will survive for years on Mars than it would be to send one to the depths of the Pacific, too.