From the Travel section of the Torygraph:

The ban comes ahead of the peak season for travel – the month-long Saga Dawa festival began on Monday – and is likely to be seen as a response to growing unrest at Chinese rule.

Last month two Tibetans set fire to themselves outside Jokhang temple in Lhasa, a Buddhist shrine that receives thousands of visitors each day. Although at least 37 people have carried out similar protests since March last year, it was the first recorded self-immolation attempt in Lhasa, a popular destination for foreign tourists.

Several Beijing-based tour operators have since claimed that the Chinese National Tourist Office (CNTO) has told them to stop taking foreign visitors to Tibet indefinitely.

Just another reminder the China still is a fascist state.