The Kodak Eastman format of 35mm film overtook the technicolour process because it was very cheap and quick, with relatively good results--relatively speaking, of course. The technicolour film process literally uses layers of dye to produce the resultant image (to put it simply, of course, though it is more complicated than this); Kodak stock uses chemicals, and of course the life of a chemical agent is much less compared to the life of a dye-soaked film strip.

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The Star Wars restoration process began with a 10-bit RGB high-definition scan of the original negatives. [...]
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I'm not very familiar with film restoration, so can anyone please explain why 24/32 bit scans aren't used for this?"

Each channle gets 10-bits each, so you end up with 30-bit colour space. This is (or was, i should say) the limit of digital video technology. 12 bit technology is now common.