Because Org's the center of the Horde universe, and there are no Engineering trainers in TB.

Even though I already know the answer, I go through this mental weighing of pros and cons almost every time I send my B-smith back to the old world.

Org has (soon to be "had." yay!!) the absolute worst anvil/forge location. TB has the best, with UC a close second. If there's blacksmithing to be done, Org loses.

Org and TB are tied for the most convenient AH/Bank/mail setup, with Org nosing slightly ahead since it's sometimes hard to get one's bearings in TB. UC has recently pulled ahead with the new mailboxes. So, this category is effectively irrelevant.

Here's the deciding factor between TB and UC: portal location. When you port to UC (or Org) you can mount immediately -- heck, sometimes you're already mounted -- and then it's a short hop, skip, and a jump to the AH. In TB, you end up in that Spirit Cave and can't mount until you run out, and then you have to guide your mount across a narrow bridge and through a narrow building.

For B-Smithing, UC wins!
(well, until 3.22)

I started Alliance and when I rolled a Horde engineer, I was completely flummoxed by the lack of smithing tools right near the AH - I kept thinking for months that there must be some somewhere close, but that I wasn't looking in the right place. Although I no longer play Horde, I'm glad Blizzard did this for the sanity of those I hope to kill.

Well, unless there is a change I missed, Ironforge and Stormwind had poor placement for their anvils (especially in the pre-mount days).

And lets not forget Darnassus with their "Anvil? What's that?" prior to 3.2

Personally I'd just be happy with mailboxes near trainers/forges