While other organized play programs (most of them, really) offer adventures only digitally, this is a bit different. The Encounters adventures are also the Sundering adventures, serving as a transition from the current 4E Realms to the D&D Next Realms. They act as a transition for gamers, but also communicate the strategy Wizards is taking. And, they are an excellent way to help gamers transition toward the style of play (more open, more creative, more DM and player interaction) while encouraging gamers to use the new edition.
Not having the rest of the Sundering adventures in print is a bit strange as it departs from that (seemingly very sound) strategy. These two adventures were among the few things on a store shelf that actually were related to the new edition.
They have also so far been really excellent adventures - among the top adventures released in the past decade by Wizards (I would argue they are the best after Gardmore Abbey).
But, there could be many valid reasons for the switch. One possibility is that if the rules were changing enough (they are going through post-playtest development) it could have been decided that a digital product would provide more flexibility. Or, maybe there are new plans for organized play and the print products will shift accordingly. It really isn't clear. Chris Tulach suggested on Twitter that this was about ensuring the most recent playtest could be supported without program interruptions. That doesn't tell my why they would decide to not support stores and try to generate sales. Maybe the first two adventures weren't selling well? That would be really shocking, and sad for the hobby if that's the case. If those adventures won't sell I'm really not sure what would. So, my guess is that it wasn't sales related, that it was carefully considered, and that something else will be released and it will all make sense.