I just finished Steps of Torment for the first time on Veteran, after putting it off till level 62 (I couldn’t resist Valbury and a few BoC runs after finishing the main story.) I noticed that the enemies’ levels were very low, 50-55 more or less, although the one hero I had to kill for a bounty was level 64. I breezed through the dungeon (and was very disappointed with the Transcendent chest. 
)
My question is, why is there such a massive gap between my level and the enemies’? BoC scaled above level 60, and I think Valbury did as well.
Probably because you should have done it at a lower level. If you’re massively overlevelled for an area then you gain little XP and loot drops aren’t very good. I’ve not survived the lower levels of SoT, but I would guess doing it around level 35-40 or even 45 would have given you better drops. At level 62 you should be well into Elite difficulty by now so can’t really expect good stuff to drop in Normal Veteran.
So would it scale to a higher level in Elite? Also, why is it the only challenge dungeon with such a low level cap on normal/veteran? Just curious.
Yes, it will scale higher in Elite difficulty. As for the low level, it comes earlier in the game so wouldn’t be as high as Bastion of Chaos or Port Valbury. SoT’s in Act 2 while Port Valbury is in Act 3 and Bastion in Act 4.
On a straight playthrough I reach SoT around the 25-27 level iirc so scaling all the way up to 50-55 is really quite high and gives you some leeway to build your toon up before tackling the lower levels of SoT while still giving you some good fights. But as an example my L63 Conjurer has reached the Twin Falls Elite portal so has done normal levels of SoT twice, once in Normal and once in Elite.
Your toon is too high to benefit from staying in Veteran, move on to Elite. If you don’t you’ll hit the level cap of 75 before you’re very far into Ultimate and won’t be able to gain any more XP.
Honestly I’d say just save all your keys until ultimate. You don’t really get a terrible lot of good stuff from spending it on vet/elite, at least not anything you can’t get from random dorks that don’t charge admission.