My (reasonable) guess is that Kris is proud of his code and would hate to share it.
And no wonder! Marvin is clearly superior over the likes of iBooks or Kindle. Gigantic multi-billion corporations like Apple and Amazon should be ashamed of what inferior software products they're offering in the guise of e-book readers.
Also, I understand Marvin 3 was a complete code re-write from Marvin 2, so that we, actually, already have 2 separate Marvins. A complete code re-write has its advantages, but also disadvantages. See all the missing/lost features from Marvin 2 that still aren't available in Marvin 3!
I have heard of solutions where there is a "fork" in software development at a certain stage, for whatever reason. Isn't that what happened to OpenOffice a few years ago, so that today, we have both
OpenOffice and
LibreOffice, and users can choose either package? And a few years earlier, I think the same thing happened with
Mozilla, where it split into
Firefox and
SeaMonkey.
So, just in theory (but it may be unacceptable to Kris, and the very idea might offend him), Kris could retain his full rights to Marvin and keep Marvin to himself, for future development at a slower rate that is convenient to him. But, at the same time, if he shared the code, there could be a spin-off product from
Marvin (called
Malvin for all I care),
where volunteer developers like yourself might attempt to improve/further develop the code.
I'm not optimistic that this will ever happen, but just in theory, it's possible.
By the way, Kris originally also mentioned he'd love to create a Windows version of Marvin. (Prior to that, though, it might be smarter to go for Android as the dominating mobile platform nowadays; Moon+ Reader Pro is certainly an excellent app, but not quite as good as Marvin, so it can certainly be "beaten on its home turf".) I also have a few friends who ditched Marvin precisely because it's not available on desktop. So, one of my friends switched to Google Play Books stating this as the primary reason: that at any time when he's at his Windows desktop machine, he can just fire up Google Play Books and see all his highlights and annotations right there before him, so he can process them right away without any manual export operations the way Marvin makes them necessary.