You might start by asking a Catholic; to ask what's-his-name is rather like seeking from Hamas the reason why Jews cling so fiercely to Israel.
We hold so fiercely to Mary because we appreciate so much more than the "anti-Mariolatry" crowd the greatness of Divinity, the quite astounding Mystery of God, the Most High, the Lord of Hosts, the Creator of all, becoming Incarnate in the womb of a Virgin; the "anti-Mariolatry" crowd -- whether they realize it or not -- reduce Christ to someone whose entire importance relies on His functional value to them (this "functional value" thing appears elsewhere in their thought too, but I digress).
To denigrate the human locus of the Miracle of the Incarnation with the truculent "I'm every bit as good as she is!" is vulgar, to say the least, and it denigrates the Incarnation itself.
Several claim (was it on this thread?) that God chose Mary because "she fulfills the prophecy," which lots of girls at the time could have done, as if God stumbled upon a prophecy independent of Him, made by someone else, but by which He is unaccountably bound, as if the God who, as they claim, had every detail of His Plan of Salvation worked out from all eternity, somehow overlooked planning and preparing a suitable vessel from all eternity and had to scrabble around at the last minute to find someone to answer His "help wanted" ad!
Some are unwilling to answer whether they believe Christ is God; some say they do, but their "God" seems awfully small.
I think it all comes down to Mary’s fiat and free will.
There are some on these boards who absolutely deny free will.
I can say now why would I ask a catholic? But I did do just that in the beginning, finding there answers were anything but adequate...most don't know why. Those who do varied so in the history and or reasoning's about it that it was a fruitless cause....so I went to the Vatican...it is there the RC teachings are clearly shown...and equally as much writings of the Popes... for they do set the tempo of what is to be believed indirectly and directly. The dogma's there and all anyone might need to know what catholics are suppose to believe.
I believe there are various reasons catholics hold to Mary and various degrees of worship of her. But your Popes do set the pace and example...of which the members follow.
I would suppose to a catholic I am now part of the anti-mary crowd. For I do believe how she is portrayed in the catholic faith is indeed idolatry...and completely so.
In my Christian walk I have not spent a great deal of time looking at Mary when it comes to the Birth of our Savior. Rather on His coming and what that means to every individual. Equally as much His life and mission which followed. Yes, that God became a man is astounding and has often humbled me....that Mary was selected was an event which played out in the role of His birth...is pretty much enough for me...even after investigating further.
You see the old test. spoke all the way thru it that the Savior would be coming....it's about Him...very little mentioned of Mary...in fact her name wasn't mentioned..But his was. His birth was a fulfillment of all that was written...and that is cause for greatest of celebration over Mary's role...though she is certainly honored by God at that time...she was far far more humble than those who lay claim to her worship today...and would no doubt cringe for those who have exalted her above her son...and they have and they do.