Standard SQL-92 is represented by >; its inverse is!=. Both test for values, which NULL is not because it just serves as a placeholder to indicate the lack of a value. Because of this, the only valid predicates in these circumstances are IS NULL and IS NOT NULL. It is not just SQL Server that exhibits this behavior. All SQL dialects that adhere to standards function in the same way.
Note: You use IS NOT NULL to compare if your value is not null, whereas you use > 'YOUR VALUE' to compare if your value is not null. I can state if my value is NULL or NOT NULL but not if my value equals NULL or not. If my value is anything other than NULL, I can compare.