Because town portals kind of behave like a presence at that location. If you go to town, any effect (cold, poison, prevent monster heal, etc.) will remain active on monsters around the TP.
That's also why Baal's clone won't disappear if you leave the TP next to him. But if you open the TP far enough and go to town, he will have disappeared when you return from town. ;)
Dang I didn't even knew that!
Congrats on Dclone P8 with a bowazon lmao