The International tournaments hold massive prize pools but that wasn't enough to persuade the original host country of TI10 and now Dota 2's biggest tournament will be held elsewhere.
Romania stepped up and will be the host country for TI10, which has been delayed already and didn't happen in 2020 due to Coronavirus issues. It was apparently these same concerns that decided the tournament wouldn't be held in Sweden, the originally planned host country, since the government declined Valve's requests for reclassification which would allow TI10 to be held in Stockholm.
It is for this reason that the biggest Dota 2 tournament of the year will be held in Bucharest instead and will be slightly delayed which is understandable, given the short notice for both Valve and Romania .
Group stage will start on October 7, 2021, while the Main Stage portion will start on October 12. The grand finals will be held on October 17, 2021, when the two best teams will face off for the lion's share of the massive $40 million prize pool.
Ticket sales are not live yet but Valve stated they would be offering more info about them soon.
Interestingly enough, Sweden is not in a state of complete lockdown and sporting events are happening in the country. It remains to be seen why TI10 was denied, especially if you factor in the massive attention such a big tournament brings to the host city, state and everyone involved.