From all I have read, Jane Parker was a spiteful, embittered woman who loved intrigue.
George Boleyn detested her, and this probably had an effect.
Also, Henry VIII was known to desire a male heir; his only son Edward was sickly and unlikely to live long.
Had Catherine Howard become pregnant, her influence woud have grown (and thus, by association, Lady Rochford's influence).
A pregnancy would also provide a safety measure for Catherine, since Henry's matrimonial track record was not good.