It's mostly the tendency of the lugs to throw up dirt and mud, and the fact that most of the time you aren't doing the hard braking in soft ground that would make tread lugs worth having, so it's really no point unless you really want soft ground traction for braking and are willing to live with your tires coating the bottom of the wings with crud all time. Most ultralights stop pretty quickly when they land anyway and don't really need heavy braking ability on soft terrain.
Big airplanes in the 30s and 40s commonly used diamond tread tires because they still operated from grass a lot, but that practice faded as soon as paved runways became the norm.
If your ultralight is a taildragger with good brakes and lugged tires, you'll end up with too much braking traction and nose it over, so it needs to be a trigear if you want to be able to do that. I think you'd find in the end that 90% of the time, having lugged tires or regular ones makes very little difference overall if you are only landing at 25 mph in the first place.