Because this board is full of Genwunners who believe Mewtwo should stay as the strongest pokemon and don't like any pokemon who is stronger than Mewtwo. These are the same Genwunners who are whining and fussing about MewBuu.
Honestly I love the new Mew and I pretty much like most of the new pokemon but I don't like Arceus. I don't hate him but I don't like him. Also I generally don't like legendary pokemon because I don't like how over the top most of their descriptions are with making mountains and creating oceans or bending space. Mewtwo and Genesect on the other hand are manmade and just very powerful.
Personally he just looks odd to me. I like the head but the legs and lack of feet doesn't look right to me and his body seems longer than it should be.