Why does this region fit NMR spectroscopy technique?
The insert in the Ld ortholog occurs in a beta sheet and suggests the presence of additional strands in that sheet compared to the human variant. The structure of the human protein has been determined by crystallography. It would therefore appear that it should be possible to determine the structure of the Ld protein the same way (via crystallography), provided a crystallizable construct is identified. 

Based on the available reference provided, the insertion feature from LdRab5a appears to make the protein unstable in vitro and difficult to crystallize, leaving NMR as the only good alternative for a structure determination. However, it seems the insertion feature also rendered the protein too unstable to be able to complete long NMR experiments, so the authors chose to remove the insert in order to stabilize the protein and perform NMR.