Why wasn't this film called The Queen and Di?

Because it's not really about Diana. The Paris accident and death of Diana is just the backdrop to the story of the Queen in those months: how she dealt with a new Prime Minister, how she managed her role as monarch, how she dealt with her family. It could have been anything--a war, a constitutional crisis, a royal scandal.
The Diana incident just provides some historical context to the story, which is primarily about the Queen. Hence, the title.

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I agree with you. Actually I thought it should have been called something like the "The Queen, The PM, and the Princess" clumsy as that is. It really is a dramatic balance between all three of them - how each one pushes and pulls at the other in impact. Such subtle interactions - this movie handles everthing so expertly.

Don't worry, someone here got it.

http://www.youtube.com/watch?v=7cG79lGWyL4

*golf clap*

People, the OP was making a pun out of The King and I.

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