One of several reasons:

“Theresa May refused to take part in a US operation to protect ships in the gulf, the US's national security adviser has said

John Bolton is currently visiting London and said the UK's original preference was to take part in a European operation.

But new Prime Minister Mr Johnson has reversed the previous government's policy and has now signed up to the US mission, as reported by The Sun.

Mr Bolton said: 'We were pleased PM Johnson's government has agreed to participate in Operation Sentinel, which reflects a change from the prior Government…”

https://mol.im/a/7350905

Well, to be fair, Theresa would have been busy having Parliament hold another vote on the Brexit Capitulation Deal.

First things first.