Well, Earl Spencer may have got a huge chunk of money from the Daily Mail for the serialization of his book, but with the expenses of ex-wives and running Althorp that's run the Diana course a long time ago, he'll be back in the financial dumps in the near future. We know Meghan spends like a fool so it wouldn't work.
In addition, Charles Spencer and Meghan Markle are too much the same personalities, there would be huge clashes. She can't lead him around by the nose like she does with Harry.