Because of Merz's border controls: Without this document you are no longer allowed to travel on Flixbus

On the very first day of his term as Interior Minister, Alexander Dobrindt ( CSU ) instructed his ministry to increase controls at Germany's external borders and to reject unauthorized entries – including asylum seekers. These measures, intended to limit immigration, also impact the movement of people and goods.
Coaches like those operated by Flixbus , the European market leader, are particularly affected. Flix SE, which owns Flixbus, has approximately 3,000 employees across Europe. Coaches are intended to be the preferred means of transportation for asylum seekers. Their inspection takes a long time and can lead to delays in travel.
In an interview with Buzzfeed News Germany, Flixbus CEO André Schwämmlein now says: "We are making every effort to minimize the negative impact of police checks, for example by allowing enough time in the timetables to avoid possible delays as much as possible."
Flixbus plans to increase security on international routes by requiring passengers to present a valid ID upon boarding. Without these documents, passengers will not be permitted to board, explains the Flixbus CEO: "This will enable more efficient and smooth processes during police checks on long-distance buses."
However, a company spokesperson points out that, under EU law, the thorough verification of ID cards is reserved exclusively for the authorities. She therefore calls for closer cooperation and their presence "not only at borders, but also at international train stops," similar to airports and train stations.
Berliner-zeitung