1. There isn't a sound logical reason for that. Obsolete != Superseded.
2. A mod can become obsolete because of game changes that simply render it irrelevant (i.e. mod added Boss Rush but game update introduced dev's own version, in this case mod isnt unsupported - it is obsolete). Using "unsupported" in this case is invalid, it indicates author no longer wishes or can support which is is wrong - there is simply no pragmatic and logical reason to do so.
Setting mod to OBSOLETE should have an option to redirect to new mod that supersedes it, but it should not be required