I have a feeling many of the same technical reasons apply to the Watch Dogs downgrade.

Those initial trailers simply aren't open world. They are in engine, but they're specifically scripted in a way that allows you to go nuts on graphics. While it works when building the trailer, sometimes trying to bring that to a full scale open world causes problems. People want optimization? This is how it works. If the game isn't running good, you're likely going to need to downgrade stuff to improve performance.

Obviously there's always an effort to work on improving efficiency of the hardware in consoles (doesn't really apply to pc) but that kind of stuff takes time, and the gains aren't always completed fast enough to deliver the quality you originally wanted. This is particularly a problem early in a console's life. If it's the first game a studio has made on a console, it's probably not going to come close to its full optimization potential. And yes this is true regardless of hardware, even PCs if they were a locked spec.

As for questions of why that stuff couldn't still be included in a PC version even though it didn't work on console, the developers explain that working on several versions means you need to focus your attention in certain areas. It may not be worth it to create extra features and better quality models, lighting, etc, if they're only going to be used on a small portion of the market. You need to focus your attention on where the majority of gamers will be.

This is one of the biggest reasons I'm primarily a console gamer over pc. Besides the obvious benefits of plug and play and optimization, the better hardware on PCs doesn't usually result in much of a difference graphically compared to the console versions. Yeah you can get better resolution, AA, frame rate, etc, but those aren't the most important aspects of gaming to me.