I don't get that engine in the center of the picture. Why would it be on the inside of the capsule?
would say two factors there are obvious:
Volume and
CoG
The motor needs a specific geometry to get the right thrust curve. you can of course try to fit a SRM into a more complex geometry, but that not just increases costs for manufacturing, you also can't just buy the motor from the shelf.
So, the motor has to be installed somewhere where enough room exists for it.
Next, once you have an abort, the motor will burn fuel, get lighter and the CoG shifts. If you have the CoG of this motor near the CoG of the capsule, you have less problems there. Would you install it under deck, the CoG would rise quickly during the abort and controlling the capsule would get harder.