A few things occur to me. I think they want to give it a spotlight before E3, because they want to give E3 to the other 4 heavy hitters. They want to give Days Gone a "deep dive," too, but they know another E3 showing would be overkill. Plus, the game doesn't seem to show well at E3, for some reason. Like Busterman says, it doesn't have the "wow" factor that makes it stand out. So maybe they think an extended GI feature would do the game more justice than an E3 showing -- which I think is true.
As for why they are showing it before E3 rather than much later, it could be several things. One is that if it didn't go before E3, people would wonder why it wasn't there. Is Sony ashamed of it? Is the game in trouble? You know how people speculate. The game is having a hard enough time in the press without adding those sorts of rumors. Another reason (probably a big one) is that they are trying to correct misperceptions about the game -- they want to help people get past first impressions ("eh just another zombie game") and have a better understanding. They want to do that as soon as possible, which is smart -- you want to try to correct perceptions when they are still soft and maleable, rather than waiting until they have set like concrete. Another reason could be that they are ahead of schedule (which is what I've heard), so they are confident enough to show it in the current state, even though it's got 9 or 10 months before it releases.