Dear sister Andrea,
Here’s the promised answer to your question “Why did the Catholic Church have to make the Marian doctrines necessary for salvation?”
My initial response was:
Permit me to explain further.
Please take note of the proscriptions for the Marian Dogmas. It was very carefully worded. Do you see the words: “Let him be anathema”? If not, that is all you really need to know.
The proscriptions for the Marian Dogmas are not anathemas, but excommunications. The difference is this: Excommunications exclude you from the Church Militant (i.e., the Church on earth). In distinction, anathemas exclude you from BOTH the Church Militant AND the Church Triumphant (i.e., the Church in heaven). Of course, through the power of the keys, the Church always has the authority to revoke excommunications and anathemas.
No one is being damned to hell, according to the proscriptions contained in the Marian dogmas. In fact, the Dogma of the Assumption does not even contain an explicit excommunication! That lack of belief in these dogmas sends anyone to hell is just a polemic misrepresentation by some Catholics to inculcate fear for not believing in the dogmas, and many non-Catholics to inculcate doubt in the Catholic Church.
Some will argue, “The Church has not formally distinguished between excommunications and anathemas for a long time, so even though the Marian dogmas don’t contain anathemas, the excommunications mean the same thing.”
The truth of the matter is that this loss of distinction only occurred in the early 20th century. The Code of Canons of 1917 was the first formal document to drop the distinction. So when the IC was dogmatized in 1854, the distinction between an anathema and excommunication was very much in force. The fact that the dogma of the IC did not contain an anathema is very highly relevant (also consider that Vatican 1, which occurred after the dogmatization of the IC, used anathemas). As far as the dogma of the Assumption, if you read the proscription carefully, you will discover that it does not actually even contain an explicit excommunication (as already mentioned).
Further, both dogmas contained clauses that explicitly took into account the mitigating factor of invincible ignorance. The proscription of the IC applies only to those who “obstinately maintain” their position against it; and the proscription for the Assumption states the same with the words “willfully deny or call into doubt.”
I hope that helps.