Why was Sir Philip so interested in marrying Kit?

From the text:

"The man's a brute," she said viciously. "He doesn't care twopence about me, really—treats me like a child. All he wants to do is to lay hands on my estate. That's why he tried to fix up a formal public engagement before I was old enough to realize how serious it all was. He thought I wouldn't dare break it off and as soon as I was fifteen or so he'd marry me and take everything."

because of her money

Because she was an heir to her parent's fortune.