Because Model 3s are extremely late and we think (and so should Y orderers) that the longest waiting customer should be served first (on average of course - 1 or 2 isn't an issue, but when it becomes en masse, it becomes an issue). It's not irrational.
Would you be cool in line at Woolies if thousands of people were allowed in front of you simply because they purchased a specific variant of milk? I would wager absolutely not.
Tesla should have cleared what they started with Model 3s before they released Model Y with a 2 month EDD. There is a fear Model Y will be prioritised over Model 3s and eat into shipping capacity and Australian staff's capacity to physically perform deliveries.
In essence, Tesla were already struggling badly to live up to targets and they went and chucked 10s of thousands of extra orders into an already underperforming mix. Which is, for lack of a better term, dumb.