I think BS hits it at the end there - the business perspective:
From an ISP perspective, being part of a "bulk order" implies they don't care exactly when an individual gets converted over, just that it happens soonish.
This lets Openreach do the planning with "economies of scale" in mind - to pool together a chunk of the bulk orders on one exchange, so they can put the right staff into place (especially the extra staff brought in, volunteering overtime). The exchanges chosen can be cycled over a period.
As Openreach can focus staff onto these orders, the cost per line is cheaper ... and can be passed on to the ISP. ISPs are then incentivised to move most people over through the bulk order ... which in turn helps Openreach plan teams of engineers better.
Ultimately, the bulk-order process is a win-win for Openreach, ISP and customer.
Otherwise the changeover order is specific, and needs to be performed by "regular" staff, following "equivalence" rules.