I did research and these are the reasons I can find. Celesteela is based on the princess from The Tale Of The Bamboo Cutter, with arms like Bamboo sticks. Celesteela is found in Maile Garden, a grassy area. Also, Bulbapedia states the biggest part to this "Despite being a Steel type, Celesteela's body is plantlike and can absorb nutrients from the soil." And even the French name for it is Bamboiselle, which stems from Bamboo. So despite all of this, its just not grass because Steel for being a space shuttle, and Flying because it's said to fly into space. They didn't give it Levitate because they don't want to give UBs abilities beside Beast Boost.