WHILE Statement Never Ends

Why does this query never ends??

Seems that parameter @Sucursales is always the same number. When the query loops in the while statement does it set @Sucursales to the result in the select statement (in red)? Or continue looping from the while statement?

DECLARE @Sucursales INT

SET @Sucursales = (SELECT COUNT(*) FROM CatSucursales)

BEGIN

WHILE @Sucursales > 0

INSERT INTO Compras(Fecha,NumOC,CveProveedor,Importe,CveDepartamento)

SELECT

Fecha,

NumOC,

CveProveedor,

CONVERT(money,SUM(Cantidad * Costo)) AS Importe,

CD.CveDepartamento

FROM

ComprasDetalle CD

INNER JOIN

( SELECT

CveProveedor,

Nombre,

CveDepartamento

FROM

CatProveedores CP

INNER JOIN CatDepartamentos CD ON

CP.CveSucursal = CD.CveSucursal

WHERE

CP.CveSucursal = @Sucursales

)VirCP ON

CD.Proveedor = VirCP.Nombre

AND CD.CveDepartamento = VirCP.CveDepartamento

WHERE

Fecha = DATEADD(dd, DATEDIFF(dd, 0, GETDATE())+0, -1)

GROUP BY Fecha, NumOC, CveProveedor, Nombre, CD.CveDepartamento

SET @Sucursales = @Sucursales - 1

END