Does the bathroom just have a shower at this point? Is there a lav? If so, does the water there get hot? If it is just the shower, it could be as simple as the shower valve. Many have and adjustment to control how hot the water can come out at. Just might need adjusting. Could be other things as well, but we need to figure out if the bathroom isn't getting hot water or if you just aren't getting hot water out of the fixture(s).
I doubt the problem is heat loss. The pipes should be in interior walls (especially in a cold area). Even if they were outisde, water at 140F flowing at a couple GPM has quite a bit of energy and it would be difficult to cool it off much with a striaght run of pipe in fairly still air.
If the lav isn't installed yet, but the valve are there, you could run the water into a bucket and see if if gets hot.
Other possibilities for hot water not coming out hot would be a crossover in one of the shower valves (maybe in a bathroom that isn't finished yet) allowing cold water to mix with hot.