The Xtracycle design is an open standard, however, all dimensions and setup are based around using steel is a material source. A lot of bicycle manufacturers are focusing on aluminum, probably because it's easy to market the benefit of lighter weight than steel. This causes steel bikes to be placed at the low-end (cost-centric, low-precision builds) high-end (strength and durability-centric, such as cross bikes, touring bikes, and the niche market of cargo bikes) of bikes without much room for the average rider to buy one. This is why you see a lot of smaller brands like Surly, Masi, Spot, and others focus on steel bikes, though similarly equipped to aluminum counterparts, they tend to be slightly higher prices because of economies of scale.
One can't (or at least shouldn't) use Xtracycle's open-source (no need to license) design when building a bike out of Aluminum.
A second problem is that large brands have a hard time building product that is supported by a smaller brand. For Trek, in order to build bikes to the Xtracycle platform (with steel), they would also need to either reproduce other pieces (such as the H-Racks and freeloader style bags) or order them from Xtracycle and stock their warehouses with Xtracycle product so nationwide retailers have reliable access to add-on components. Perhaps with Xtracycle being relatively small, they couldn't get a large enough supply (at a price acceptable to Trek).
Given that Gary Fisher was given an Xtracycle by the X guys some years back, I'm sure the idea was explored. I had high hopes that Trek would produce an Xtracycle, but I am happy to see that at least they are offering a cargo bike.