Comment by Arnt

In practical terms:

Back in those days, people would store a mixture of ASCII and other data in the same database, e.g. ASCII in some rows, ISO-8859-9 in others. (My bank at the time did that, some customers had all-ASCII names, some had names with ø and so on.) If unicode were only mostly compatible with the combination, it wouldn't have been safe to start migrating software that accessed databases/servers/… For example, using UTF8 for display and a database's encoding to access a DBMS would have had difficult-to-understand limitations.

You can fix all kinds of bugs if you're able to disregard compatibility with old data or old systems. But you can't. And that's why unicode is constrained by e.g. the combination of a decision made in Sweden hundreds of years ago with one made in Germany around the same time. Compatibility with both leads to nontrivial choices and complexity, incompatibility leads to the scrap heap of software.