Because Rielly ends up being better. Every D who we try because Rielly is obviously the problem with the Marner run PP puts up worse to significantly worse PP numbers.
We were only ever going to improve on Rielly by bringing in an elite offensive defender with a shot that forced the PK to cover the point, or by going 5 forwards (but Marner running the point had the same problem Rielly causes up there because Marner has as bad a muffin as Rielly does)
Here's the list of D who have been more productive than Rielly on the PP over the last 3 seasons: Makar, Hughes, Hutson, Sergachev, Hamilton, Fox, Josi, Hedman, Bouchard, Barrie, Dahlin.
That's the list.
Rielly's ability to distribute the puck and hold the blueline to keep possession in the zone on the PP is massively underrated by some people. There's probably a limit on how good your PP can be with him on it, but there's also a floor his presence is going to keep it above as well.