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the Doctor visits present-day Earth quite a lot.

For example, in series 6, he visited 2011 or 2013 Earth in "The Impossible Astronaut", at least once after "A Good Man Goes to War", "Let's Kill Hitler", "Night Terrors", "The God Complex", "Closing Time", "The Wedding of River Song", and the Doctor, the Widow and the Wardrobe.

But some seasons he visits more often than others—the 3rd Doctor spent all of season 7 there, while the 6th Doctor didn't show up once in season 23.

They don't usually explain why they've changed things from one season to the next, but often it's just for a change. If the producers or writers feel they've done too many present-day stories recently, they'll do a lot fewer the next season. It also probably depends on the companions—when you're traveling with a girl from the future and a man from the past, or a Time Lady and a robot dog, there's a lot less reason to show up in the present day.

In the case of series 5, we do know why the Doctor only visited modern Earth a few times, and all during a 24-hour period in a small town: Because of the frequent public alien invasions over the past few years, Steven Moffat believed that present-day storylines break suspension of disbelief, because people don't react the way they would in the real world. The cracks-and-reboot storyline that he spun during that year solved the problem, which is why we got more present-day storylines in series 6.