but the cynic in me wonders whether prosaic factors like cast availability might sometimes be more pressing reasons.

Why be cynical about it, Peter? After all, in his day, Shakespeare was a theater man, and had to put on productions in the real world, as well. He, too, had to worry about concerns of cast availability, and which actors had the skills to play which parts.

He, of course, had the luxury of working with/leading an ensemble cast of players... the same actors over many years, and got to know them well, and they, him. His was really the first era of English drama that created the concept of "stars" of the theater...

In any case, it's how well you adapt artistic vision to prosaic concerns that make art "high art".

And good for you and your daughter for exploring Shakespeare via the stage... that is how his work was meant to be experienced -- not hunched over a book, squinting at the glosses in the margins, trying to understand what these funny words mean.

It was when I was studying Shakespeare (if my memory serves me well) at age 16 or thereabouts that my English teacher (Don Fried) said something I've never forgotten:

"Reading a play is like looking at the blueprints of a house. Seeing a play is like walking through the rooms."