Mary
The simple answer is because it does.
The more complicated answer is that the DWP have four weeks from the date of the hearing to decide whether they want to challenge it further, the department that looks at Decisions almost always has a back log of results to process, also, depending on the amount of back payment, there may need to be additional authorisation to release the funds.
It's worth phoning the DWP to make sure that they have the Decision Notice, but I'm afraid they are not going to do things any quicker.
Gordon