They seem to have a very strange business model which enables the middlemen (Diamond, comixology etc) and Comic Shop owners to survive, whilst only distributing, not creating anything. The fact is that they're completely unneeded in digital terms if the Companies had even a few semi-capable techies.
I think we have a big part of our answer right there. I would be willing to bet that pre-arranged contracts with a company like Diamond has the most impact on how Marvel is handling digital at this point. If Marvel were to release digital comics at a lower price, it would undercut Diamond. I would be willing to bet that there are stipulations in their contracts that prevent this from happening, in which case it's at least partially out of Marvel's hands until the contracts expire or are renegotiated. The same thing happens, with TV shows and services like Hulu, in determining what is available free, or by subscription, or web only, or not at all. Even if the network and distributor are for offering a program free with commercials, if the production company's contract stipulates web only, then it will be available web only. It defaults to the lowest common denominator, which in the case of digital comics is likely Diamond for the moment.