You already own the licenses, so you don’t waste the money you’ve spent and you don’t need to re-invest in purchasing software.
Your people are already familiar with the software, so further training would be significantly less costly and would be done faster.
After a few months of joint efforts, you have established a working relationship with the vendor, so you can work together much more efficiently.
Software vendor consultants and the client’s implementation team do not need to re-examine the client’s business processes.
You’re not starting from scratch, actually. All the project team needs to do is make the necessary corrections to an already existing project plan from the previous implementation—and they can start on this almost immediately.
As the majority of the preparatory work has already been done, the overall cost of the project would be lower than that of any alternate system implementation.