In the US an ultralight has to be under 254 pounds and does not require any training or licensing to fly. In most other countries an ultralight is more like the sport class in the US.

At 254 pounds you want light and simple. FBW requires actuators. And controllers. And sensors. And power. And then you have to double up everything for redundancy. You would easily eat up 50 lbs or more making a reliable FBW system and it would cost 3-4x what the entire aircraft cost.

Even with normal light aircraft a FBW system is both unneeded and overly complex. Cables and pulleys are extremely reliable. They are inspected every year (along with the engine and various other pieces of the aircraft).