So, one of the things I really like about War of the Ring (and I understand this holds true for the SBG as well) is how much more important movement and maneuvering is in those games compared to the Warhammer games. WotR really feels like you have genuine decisions to make regarding where to move and who to try and attack and how to attack them, unlike WFB and 40K where it seems like games are won or lost in the list building phase and once the game begins the "choice" of where to shoot or who to charge are all pretty no-brainers.

My question is why? What mechanical elements about these games make movement so much more important and the games so much more dynamic?

Is it that formation's don't get locked into hand to hand combat, so you can keep moving and maneuvering throughout the game? Is it the larger distances that can be moved, with an at the double my goblins can fairly reliably move 18" before charging? Is it the significant flank and rear attack penalties in a fight that make it so appealing to keep trying to maneuver? Is it the random charge distance making? Is it premeasuring?

What do people think?

I think it's something to do with your opponents in Warhammer and, to a lesser extent, 40k. 40k is largely about cover and the deployment within it. Warhammer is a game where it centres around movement. The bonuses and penalties of flank/rear charges (as well as dissuading people from charging for various reasons etc) are the biggest part of the game, IMO, far more so than -3 or -5 attacks in WotR. Also, the distances aren't that much larger (Warhammer has marching and then charges are 2D6" plus your movement value, which can be up to a total of 22" charging - Dwarfs can get up to 15" charging too).

Honestly, I think that it could be that you've not found the maneuvrability of WH yet. It's a little out of whack at the moment because of the change to 8th edition (so Skaven are rock-hard and Vampire Counts suck unless played in a specific way), so list-building is a factor atm, but no moreso than WotR...