The start of the month is tied in to the whole targets/budgeting feature. My understanding is that this is currently being completely rewritten, so they are not fiddling about with the existing feature (even for something that seems as trivial as setting the start of the month – there are always implications for such changes that have to be worked through). Even if you could set the start of the month, there are issues with the current Targets implementation, such as the fact that being paid back for shared charges or expenses doesn’t decrease your spend, that make it difficult to use for anything other than the most simplistic budget.
For an idea of where this is going, see here:
I think it is likely that Pots had to come first, as it is pretty foundational to the architecture. When the revised Spending tab comes along, we will want it to work smoothly with Pots, so this way they can be integrated with the new Spending features from the outset.