Some of the reports argue that it was a placement issue, the books ending up in the comics/graphic novel section rather than the YA or manga ones. That was certainly true here in Singapore, and maybe it was an unfortunate result of the hybrid nature of the line.
Personally of course the worry is that, whatever the marketing or placement issues, some of it comes down to the quality of the stories themselves, in this case the art - would Regifters have done better with better drawings? Or if it had been done in a different style?
There've certainly been some who thought that different creators might have fared better - getting more YA authors involved for example, or having more female creators.
I'm sure though that Shelly must have thought all these things through, and my sense is that any set of decisions would have been criticised at some level - if they had gone with more YA authors, there might have been complaints about not making more use of actual comics creators who understood the craft better; if there were no male creators involved they might have been accused of a sort of reductive reasoning that you could only create comics for your own gender; and I'm pretty sure any name other than 'Minx' would have ended up with problematic connotations at some level or another; and so on and on.
Having said that, I think the debates over the reasons for the demise of the line is interesting - leastwise in helping everyone figure out what exactly makes Manga and YA fiction so successful in the first place. How much is down to marketing, to the quality of the stories, to that ineffable thing that helps a Viz, TMNT or Harry Potter catch fire like its nobody else's business?
And maybe some of it really is a mystery - how on earth did [insert name of your most-hated movie/singer/band/book/comic] end up making millions whilst [that excellent movie etc you really liked] ended up sinking without a trace? As comics creators i guess all we can really do is try and tell the stories we think are worth telling and hope that the jungle of the marketplace out there somehow thinks they're worth reading too.