A few reasons, really. It's easily obtainable content that never runs out, it's easy XP and there's no needing to go to different zones like in regular mission arcs.
The drawback on PI radios is you face the same enemy groups over and over (because people skip caves for CoT and generally avoid Carnies) so you just end up fighting Arachnos and Council, and the missions are very short.
Kallisti Wharf repeatables i've found to be much more fun, with Sky Raiders, Skulls, Awakened, UPA, Warriors, Freaklok, there are more enemy groups to keep it varied, they're a bit more challenging, and they're on better maps (all my subjective opinion of course). For me at least, there is no up side to PI radios over KW repeatables now.