A few thoughts around this (pure speculation all)
- Issaries is the god of equal exchange. Not that everything he does has a balancing or reflective factor, but I think it would not be crazy to say that when he traveled into the West, and sought paths and knowledge to do so, that this enabled, encouraged, brought into being, or whatever a travel into the east. Or, just as likely, the reverse....not that the Heortlings would believe the former option!
- My personal opinion is that what is now Wenelia is the stitched together rubble of what was much more in the god time. The cosmic net drew in the pieces and bound them together as much as they could be bound, but part of the areas problems is that it literally is missing parts of itself. (OK, I think this is true of most of Glorantha--you can even observe where the edges were in Fronela as that is where the ban came into place--just much more so in Wenelia). Because so much was missing, the people/religion that may once have celebrated the voyage into the east were either lost, or could not magically function because too many steps of that trip just were not there anymore....too many holes in the story for it to hang together. So what Castlemain really did was to re-write that story, aligning the remaining pieces into a coherent whole. He didn't know he was doing it, mind.
- Clearly the Wenelians knew that the Darkness was done, but in some ways a lot of their societies never adapted to the dawn, even once they knew it had happened. They were tuned to doing whatever it took to survive at a very local level, and never absorbed any lessons of working together for something greater. In this sense Castlemain is like a cultural hero, showing the way that people can work together in a greater way. But it is much harder to undertake this role in modern times.....