Only big companies (I think) will have a Bacs facility. And then the payments could even be centralised in some payment office; so you may clean a property in London, but the payment office could be in Glasgow.
So, the relevant paperwork has to be forwarded to the payment office, and then they won't make paying the window cleaner the No 1 priority; we're just in with the rest of the payments. Some clerical type person will do the manual input for payment on computer, and then a manager type person will authorise the payments; there could be, for example, two hundred payments to make.
And then you've got to wait three days from when the Bacs payment was transmitted.
Problems could be paperwork getting lost or buried in a pile of other paperwork, payment staff on holiday, the person authorising the payments to be away, or have more pressing problems to deal with.