Thanks for the questions!
It's mostly driven by what our customers have asked for. Subtractive processes like CNC have a high barrier to entry (compared to additive), so we started there to make it easier for everyone to get quality parts easily and quickly. Aluminum is a great material that you see a lot in both prototype and final products across industries.
The CAD integrations, materials, and part complexity we support are constantly improving - much more to come in 2017!