Answer :
because in difrent materials atoms are more compact or less compact.if they are less compact then it will be easear for them to move
If they are in a solid state the atoms are aligned together, they are tightly packed, have a solid for, but do move they just move so slow that they just vibrate because like I've said they are tightly packed and can't move much. If they are a liquid the have a lot of space so they move around making the speed faster
