I would say because if you don't update within the first day you get that annoying badge along with constant badgering to update. Everyone expects a level of perfection from Apple. have you walked into other shops And then walk into an Apple Store? Everything has a place, everything is clean and tidy everything looks like perfection, why that example cannot be implemented into a release is beyond me.
Our phones will continue working (besides the newest models) on 10.3.3, why not wait until it's polished before release? Like I said above they wouldn't open an Apple Store that's half assed/complete same should apply here.