Usually it is a logistics issue. Consider the same question, as to why almost all marathons are usually pavement/road based.
It is very hard to find 26.2 Miles of trails.
Then consider the case of an Ironman distance race, where the prospective host city must find suitable swimming for 2.4 Miles, and 112 Miles of bike route that can be secured/closed.
Then require it to be on trails? Good luck finding such a place.
Ironman Wisconsin in Madison, WI has a multiple mile segment run on sort of dirt trails. My memories of that race from 2003 are sufficiently faint that I do not remember which segment was on dirt, but I remember the dirt trail.
Ironman Couer D'Alene, Idaho had a sort of trail/path segment on the run as well, same faded memory excuse on more details.
I recall reading that the old Roth Challenge Ironman distance race was mostly run on trails. This memory was in the context of the Hoyts, (A father son team, where the son has Cerberal Palsey, and his dad has done many marathons and ironman distance races) and a complaint that his running time was terrible due to the off road nature taking a toll on his running wheel chair he was pushing.
On a side note, the Hoyts are just amazing.