Because the word status belongs to the 4th declension, but also because oral language came first so grammar is linked to it, and the rules derive from the oral language to integrate the written language to "lock" it.
In fact, the genitive form is statūs, which is different in pronunciation of status, so again a strong link to the oral language.
I didn't create the Latin grammar, so it's the only explanation you'll get (from me), furthermore like in any language, there are exceptions...