Why has Doctor Who been Earthbound this season?

Radio Times

"Out of these eight episodes, five will have been primarily set on present-day Earth, albeit in different locations," says Huw Fullerton. "Frankly, we’re beginning to wonder if the TARDIS has been secretly clamped. So why is the Doctor so attached to Earth this series? Well, there’s a few possibilities. Some fans have wondered whether this anomaly (combined with recurring themes like DNA being rewritten and surprise teleports) hints towards a larger theme of the series, with a specific reason being planned for why the TARDIS is so set on returning to the same place."