Think about it...
The highest part of an engine is the top of the carburettor, right?
So would canting the engine to one side make a difference to carburettor height? Certainly not the way the slant six is canted, because it would RAISE the height of the ports by tilting it to that degree.
Similarly, the water pump could be set back beside No 1 cylinder if the block was vertical too.
I believe the reason was to give more accessibility to work in the engine bay along with the engineers capitalising on some work they'd done during the forties on a 60° V12.
No doubt engine length was critical because they were planning for its use in the Valiant and in place of the very long-stroke and small-bore flathead six in the engine bays of the bigger cars.
I've heard these arguments over and again with the Peugeot 404 and 504 engines, which lean over at 45° and it's only accessibility that makes any sense. Even at 45° there is no height benefit because of the carburettor... though the fuel injected models do gain there. But they came late in the peace, well after they were committed to the angle.
In both it makes a mess of right hand drive cars, by the way.