Because the battery was pretty much non-existent.
It went well BUT when I started it, it didn't recognize the new battery. After speaking to an empathy expert, (Annette), she sent me directions to go through 3 steps to fix the problem. The SMC reset was the last if the three steps and it worked.
Yes, it was pretty dirty inside so I took a little alcohol l on a q-tip and pointed brush to get all the crap out of the fan area: Annette suggested as long as I had to reopen the case, to finish the cleaning job I had done when I first opened the case.