I personally thing that while a lot of the popular Mangas has lost a lot of it's glory and has had it's story more or less, crushed, a lot of Manhwas has risen with a solid plot, good development, and interesting characters. Basically, the quality of the bigger mangas has decreased, while the quality of the bigger Manhwas has increased. That's why I think a lot of eyes has turned to the Korean Comic market. I also believe it has something to do with exploring, wanting to find new things. People always want new things whether it's a different brand of Cellphones or it's a different type of Comics. Manhwas and Mangas aren't that different per se, only the way you read them. Art differs from Manhwa to Manga just like it differs from Manhwa to Manhwa, or Manga to Manga. But, since the quality has increased (rapidly) lately on the Korean side it has become a more and more acceptable choice, or market to explore, if you may.
So, my opinion is that it's a combination of a decreasing quality in the top Manga market, an increasing quality in the top Manhwa market, a curiosity and an acceptance that wasn't there before. And most of all - availability
. Because, if people who wants to explore find the Korean Comic market, read the Manhwas, and like them. Then obviously the word will start go around that Manhwas aren't that shabby after all and before you know it, a Manhwa section was found on MangaHelpers