The reasons why are legion. First, the show fell two months behind schedule. Then, HBO halted production back in January in an effort to get the thing back on track. Series creator Jonathan Nolan and co-writer Lisa Joy used that break to hammer out the season's remaining scripts, and last we heard Westworld was going back in front of cameras in March. We didn't get another update until just yesterday, when HBO announced the show had been added back to its schedule, with a premiere happening sometime this fall.
It's a bummer Westworld has taken such a long time to wind its way through production, but my suspicion is that it'll be worth the wait. I like the creative team behind the show, the cast is great, and the footage that we have seen (which you can revisit in the video embedded below) looks delightfully strange. I'm into everything I've heard about this one, and am glad HBO's been patient with it. Better that than rushing it onto the schedule in an effort to capitalize on the hype (*coughTrueDetectiveseasontwocough*).
We don't know an exact premiere date for Westworld, but rest assured you'll know almost as soon as we do.