[DTV]

Was there ever a reason given why they can’t use Studio E with a locked off camera instead of the balcony?

The initial plans were that Studio E would be upgraded to a new set, which would then become the main studio for the channel. Given that certain weekend bulletins are still in E, that plan has clearly been delayed or abandoned, with wider plans not adjusted to make use of the resource. 

Additionally, the decision was made to move network control for the UK channel into Studio A's gallery, which is also gallery for the opt studio. While you could have a presenter in E with output controlled from A, that would be a bit more complex.