Because fighting all the bugs on hunderts of different systems at once is not smart. Adding single Features and fixing the bugs with them is smart and will get us to a final product faster than a unplayable buggy full feature version.
Can't remember when but some time ago they told us that they wouldn't update the current roadmap because they were focusing on fixing bugs. The first road map was made for us to keep track of where they were with the different components but as I mentioned above they "abandoned" this because the game suddenly required so much attention that they didn't have enough time to keep it up to date(can't update something if the devs are to busy working on bug fixes etc to give and estimate of the current progress).
What this news tells me is that they are finally at a stage where they have caught up with most of the bugs and feel ready to start sharing more stuff by being more transparent something that requires you have the time for it. Memphis might be the CM but he can't tell us anything if the other devs doesn't have time to tell him anything!
The reason to why they haven't released the "full" alpha is because of bugs if they released everything at once they would drown in bugs which would make it a lot harder to fix them. By working on the alpha with bug fixes and then releasing them via the tech demo they can work in a more controlled environment where they release updates with bug fixes, performance updates and new content until they are at a state where either all the content has been released or the game is so stable that they feel comfortable releasing the rest of the MS-1 stages, they could even end up releasing MS-2 stages at the same time if the game is stable enough and the content is at a releasable state.