Also, fashion houses often change directions when they change creative directors. They might seek to retain the "DNA" of the company's legacy, but superficially, there maybe be very little similar between the brand in one decade, and the brand in another. If you told Christian Dior that his name was going to be synoymous with skinny men's suits for a while, he'd probably think that you'd lost your gourd.
Dior retained the over-the-top luxury of the original house through those years, though, and Hedi actually did a pretty good job preserving the core of the brand. A better example would be YSL / SL since there is barely any continuity there now that I can see.