His Press Office hate him
He would look awkward with an umbrella
They don't have a gazebo, like they didn't have a plan for a pandemic
The writer thought it would be funny and the editor nodded
Zeus was angry
You'd have thought that in the UK, aka the rainiest place on Earth, that there would be a back up broadcast option in case an outside broadcast was unsuitable. Nobody looked out the window or at the forecast and said "boss, it is pissing down out there; lets just do this one at the desk, eh man?".
No back up, no assessment, no changing plans no matter how sensible.