Video game development is an unpredictable beast. Creatives come and go, ideas change and core mechanics sometimes have to be ripped out entirely at the last minute. It's a bit like being on a really cool rollercoaster, except everything is on fire and you're constantly having to put it out.
This back-and-forth is no secret either; just look at any of the major E3 demos from the past few years and compare them to the end products. Whether big or small, you'll no doubt notice plenty of changes between the two.
Whatever the reason, be it time or simply creative decisions, this approach to production has led to plenty of amazing features set to be included in the biggest gaming releases which were tragically cut out. They still turned out amazing in their own right, but they could have been even better had these elements made it into the finished products.