Because it's been a concern. Until recently, there hasn't been much evidence that he was capable of trotting without his nose straight up in the air.
(And his tweaked hip is better, as he's now moving normally.)
I tried longeing him in side reins. After trying them long a few times, I shortened them enough that he could feel them when standing with his head moderately up/out. Silly guy had no idea what was going on. He was very politely confused and even tried backing up a couple steps. It took a minute, but he figured out he was supposed to move *forward*. I asked him to walk and trot, and while it was better than without side reins, he was still pretty inverted. So I got a bit creative.
Disclaimer: I am not generally a fan of gadgets or using anything to force a horse into a frame. I tried something new on Riv in the hope that it would show him how to move without looking like giraffe on crack. So please don't read and think I'm taking shortcuts or whatever... I felt that teaching him to move more correctly as soon as possible was wise so he wouldn't develop all the wrong muscling, which would make things even harder later on.
The side reins were adjusted as long as possible, and the adjustable end (which makes a loop of leather) was put through the girth of the surcingle, coming up between his legs, and then snapped to the bit. The idea being, if he tries to stick his nose straight up, the reins would pull straight down. At first he had no idea what to do, and when asked to walk forward he did some lovely turns on the forehand. So I walked at his head, using the longe whip to encourage him to walk forward with me. When he was doing that not like a moron, I slowly moved back and had him keep walking. He got the idea and walked with his head held in a normal position. Then we tried trotting. It took him a minute to figure out that he COULD trot without imitating a giraffe, but then he really got it. He was trotting forward, nicely stretchy, and even stretching forward and down as far as he could. Then we switched to the right, and repeated the whole "you can walk, honest! I swear, just try! Please, just TRY walking instead of turning on your forehand, I promise you can do it without dying or exploding" thing.... boys.
In the end, he really seemed to figure out what I want and was such a good boy. Even when he had no clue what was going on, he was polite and sensible. What a good pony!