Why I am not able to use a column alias in WHERE clause?

Question

I have the following query :

SELECT
   ID
   ,CountEmp
   ,LAG(CountEmp) OVER (ORDER BY DATE) AS PREV_NUMBER_OF_EMP
   ,DATE
FROM (
   SELECT * FROM myTab
   WHERE DATE>=GETDATE()-90
   ORDER BY DATE DESC
) AS InnerQuery
WHERE CountEmp <> PREV_NUMBER_OF_EMP

I am not able to use the PREV_NUMBER_OF_EMP alias.