A lack of engines seems to be the answer most consistent with the stated conditions.
Stock SE5 does not require you to add any engines at all, but a colony ship isn't terribly useful without them.
Many people around here have run into a situation where they redesign a warship, and marvel at how using some other gun has given them more firepower than they expected. After building the first few engine-less ships, you learn that what seems too good to be true is