Henry VIII's succession?

Why were the descendants of Margaret Tudor (Mary Queen of Scotts) excluded from Henry's succession, instead, after Edward, his heirs, Mary and Elizabeth, the crown would p to Mary Tudor's (Henry's sister) daughter Frances, duchess of Suffolk. It can't have been due to their Catholicism, as Mary I was Catholic, and the only other explanation I can think of is because Henry was averse to the English crown ping to the Scottish monarchy, yet he had sought a union of the kingdoms in the Greenwich Treaties (1543), and he wouldn't have expected the crown to p so far, with none of his children producing heirs? So why defy the law - again (I know Mary and Elizabeth had already been ised)?