Stem cells are pretty good at moving around. If they don't want to stay in the pancreas, you won't be able to make them, and if they want to go to the pancreas, they will find their way there even if transplanted in a less invasive way.

How do stem cells move?

It seems from the article that it's harder to monitor, so by injecting in the abdomen they can monitor them using MRI and remove them if needed.

The point of stem cell treatments is to replace cells that are no longer there doing their job. It's not too relevant for the replaced cells to be in the same spot, as long as they exist and produce insulin, you are good to go.

You could argue for the location having physiological implications, for instance: it's convenient for the liver to be right after the intestines (connected through the portal vein), like this, it metabolizes large amounts of whatever is absorbed intestinally at once, before it reaches systemic circulation. This may hold true for the pancreas as well. It uses duodenal glucose concentrations to know exactly how much insulin to release, which may not be representative of the rest body's glucose concentrations. Practically speaking, it is far safer to inject stem cells in the abdomen where you have room for erscqror and can do surgery on without much trouble vs the pancreas, if anything goes wrong there, you could easily kill a patient.