There are may possible reasons, but there are two common ones. First, one year olds grow less rapidly than they did as babies and are often less hungry. This may mean they eat fewer and less foods than they used to. Second, there is a normal developmental phase known as neophobia where kids are more fearful of new and less familiar foods. Both can happen at this age and can look like "pickiness" to parents. (It's frustrating, I know, but my goal is to simply point out that there are often clear reasons behind the behavior we're seeing.)