Why GM cut a cylinder off the Atlas before the put it in the Colorado, Canyon, and H3? Why make a second motor when you already had one great engine that is almost the same?
The I6 from the Trailblazer would've been perfect in the H3 rather than I5 that was somewhat underpowered for the weight of the truck. I mean, the 3.5 (2006) or 3.7 (2007+) I5 still have more power than my 4.3 V6 and moves about the same weight, but the I6 would've been even better and would've got it moving to 60 in under 10 seconds.
The I6 would've worked better in the Colorado and Canyon because it would've better filled the gap between the base I4 and top-trim V8. As it stood the I5 was closer to the I4 in terms of performance, while the V8 was light-years ahead. I'm sure the I6 would've increasing towing capacity as well.
While a I5 was unique and cool (who all was making I5s when these trucks were made, VW and Volvo?) it didn't really have the power and performance to back up the vehicles that it was in as well as the Atlas I6 would've. There's only one reason I can think of as to why GM didn't use the I6 and that would be packaging. Either it didn't fit under the hood or it would've been hilariously cramped and impossible to work on.
Either way, it doesn't really matter now. The engine line was discontinued when GM killed the Trailblazer/Envoy and first-gen Canyon/Colorado. Which is a shame honestly, because I bet an updated I6 could've found its way into the new Canyon/Colorado without issues. Engine bay permitting.
(Note: This is only boggling my mind lately because I am looking at all three vehicles the I5 saw its way into.)