Because pH probes are notoriously unreliable. If you use it in feedback and it drifts or fails you are going to get into serious trouble with what is being put in the tank. Also as many others have experienced once you dial in reactors they are VERY stable.What I don't really understand is why I don't need to use ph probe to control reactor.
It's not to say that you can't use the probe with a controller it is just to say that there are large risks involved.
Another idea would be to contact the manafacturer and see if they will sell you one or buy a parts skimmer so that you can get one.I have missing that blue plate on the bottom of the reactor, maybe this affects. I have some scrap acrylic, I think I will try to retrofit that part.
Just other ideas that might help.