Which case does null match?
If you match the first type, that's somewhat unsatisfying. But if you only match with default, that's also unsatisfying because now we would have a problem with completeness -- if the match doesn't succeed then you have a potentially unassigned variable.
So now every match expression would have to have a default case just to handle null, but you also don't gain the advantages of static matching because everything matches default, so if you do legitimately forget a case you get no warning.
Existing switch statements are much more resilient to these matters simply because they aren't expected to be exhaustive right now. People are used to the fact that they have to deal with unassigned variables or completeness failures.
The match expression, however, I want to be more like ML where you can get strong guarantees on the "irrefutability" of a match.