Low Hanging Fruit
Engineering - Sage has traditionally focused on mathematics and has many unique features. However, already included is a massive amount of functionality perfect for engineering (SciPy, NumPy, R, BLAS etc.).
Distributed / Service Architecture - For the most part, Sage is a single process application. Some progress has been made with multicore type parallelism using PyProcessing but isn't currently very general or thoroughly supported
Engineers (for the most part) have never heard of Magma. Engineers use Matlab, Mathematica, and Maple. Matlab rules for many reasons but is a very easy target
- Its language sucks. Sage uses Python
- Difficult to extend
- Expensive - although this needs to be qualified, most schools have reasonable access due to Mathworks academic pricing
OTOH, Matlab has been around for a while and has a number of packages directed at engineers
Mathematica has had some success, has a nicer interface (notebook) but hasn't had a huge effect on the core engineering market.