Maybe because OceanGate is an American company. Then again, we are talking international waters. Joe Biden could cause a war between the UK and the US over this issue.

It's funny how no one bothered to actually answer you, or even actually do some research.

On 15 April 2012, the Titanic wreck, which lies in international waters, automatically became protected by UNESCO, under the 2001 Convention on the Protection of the Underwater Cultural Heritage, which protects cultural, historical, or archaeological objects that have been underwater for 100 years.[10]

It was not until 18 November 2019 that the treaty was ratified by US Secretary of State, Mike Pompeo, on behalf of the United States.[2] On that date, the instrument of ratification was deposited with the United Kingdom and the agreement went into effect.[11] The treaty required ratification by only two of the four negotiating parties for it to become effective.[12] The treaty requires both the United Kingdom and the United States to regulate persons and vessels under their respective jurisdictions in their interactions with the wreck.[13] Specifically, both countries can grant or deny licenses to permit entry into the shipwreck or to remove items from it. The UK has expressed its intention to urge other North Atlantic countries to join the agreement, especially Canada and France.[2]

So for the "rescue mission with hi tech vehicles" to access the Titanic it needed permission from the US also.