Why are uranus and Neptune called the ice giant planets?
They are mostly made up of ices.
The term ice here is a bit confusing. Jupiter and Saturn as gas giants because they are predominantly hydrogen and helium. The ice giants "ices" are actually heavier elements than helium and hydrogen compounds such as methane, ammonia and water. When the planets formed these materials were either frozen solid of contained within water ice.
Uranus and Neptune are the furthest planets from our sun. Because of this they have a thick mantle of icy water, ammonia and methane. Their hydrogen-helium-methane atmospheres are much more shallow than that of the Gas Giants and lack their metallic hydrogen mantles.
In addition to this, when they were forming they were far enough away from the Sun that the material that they accreted was largely ice instead of gas. Since they were so far away, they didn't have to share much of the material as the inner rocky planets did so they became very large on their own with much larger cores than Jupiter and Saturn,
This website goes into great detail about it. SolStation