As mentioned, FTB's unusual worldgen settings push copper and tin out of the traditional strip mine zone close to bedrock to encourage people to vary up their mining. This has the side effect of making copper really rare in the TF, but not unobtainable. TF's surface level for most biomes is roughly at 34, whilst the Overworld is at roughly 64.
The flipside is that most of the rarer materials are much easier to find instead.