Defendants generally don't use the lawyer they had at trial to appeal their convictions. While some lawyers can be effective both at trial and on appeal, criminal lawyers are generally sorted into trial and appellate specialists. Trial lawyers usually have expertise that appellate lawyers don't, and vice versa.
It can also be a benefit to get a fresh pair of eyes to evaluate a case. For instance, a new, appellate lawyer probably won't be emotionally invested in an argument that the trial lawyer emphasized at trial. Further, a frequent method of challenging convictions is to claim that the trial attorney provided ineffective assistance of counsel. The trial lawyer won't be able to determine whether that argument is viable, much less present it.
For more information, see our section on getting a lawyer for your criminal case.