It simply hasn't been added to the accepted list yet. Report it if you want to.
The word 'Millón' is a noun. When you want to use a noun to describe the quantity another noun like this, you use the word 'of'.
i.e. I have 3 buckets OF fish.
It's the same thing, only English has this weird exception to that rule with 'million'. Spanish does not. Of=de.
English tends to treat numbers as adjectives, so we say things like "There are 3 dogs" or "I have $3,000" as opposed to "I'd like 4 of hot dogs, please" or "Give me 3 million of dollars or the cat gets it!"
Numerous counterexamples exist where numbers can be nouns, but they are mainly expressed as adjectives.
Sometimes "de" is required for the correct answer and sometimes it's marked wrong. Why?
If the two words you are connecting are both nouns use 'de'. If the second one is an adjective don't.
Why is a million different than any other number? You can say "tengo tres dólares"?
'three millions' should be accepted (least wise as used in Britain).
Just because a way of saying something has become less popular over time it doesn't make it wrong.