Some say it's a technical limitation for heat management, memory management. While some others say it's to escape the additional tax that applies if a camera is classified as a video camera, the latter being defined as something that records beyond 30 minutes.
What's the true reason?
The import taxes are higher on video cameras than still cameras, so they artificially limit the length of videos you can record to avoid being classified as a video camera.
> Video camera recorders are subject to import duty of 4.9% or 14%, still image cameras are duty free.
https://www.europarl.europa.eu/doceo/document/E-8-2016-00127...
(Update: included better link)