It's because she's Welsh. Many English people are notoriously anti-Welsh. Just look at Neil Kinnock and the way the press went after him for being Welsh, largely contributing to his election loss.
Nothing to do with him spending most of his tenure as Labour leader fighting members of his own party then?
Kinnock did have an image problem (his baldness and his ginger hair worked against him more than his Welshness IMHO - when did we last have a bald prime minister???) but trying to modernise the Labour Party in the '80s was like trying to turn the Titanic.
Kinnock lost because people thought he would raise their taxes, repeal Right to Buy and allow the unions to take over the country again. Not because he was Welsh.