Because the whole feel of the series is something that passes over time. Stuffing it into two days may get you through the whole thing, but at the same time, you might miss the way it feels because you're reading it so fast. It might be sort of a 'living slowly is preferred here' thing. I don't know, it just feels better when you let that calm wash over you over a long period of time. You get to see how things change, but you can watch them over a longer period of time--not as long as Alpha really does, but longer than just a few minutes or hours.
That was an attempt at explaining it, and I probably failed. It's just a feeling of peace that I get when I read it, and going through it too fast would probably not be a good idea. The peace will end faster than you'd want to, or something. I read it over a few months, so I can't really tell.
And this thread has the same calming effect of the manga. It's a nice break from the regular threads on /a/.