Abraham was different, God said prove your faith by killing your son (shortened version).
With Mary, God sent the angel to tell her she will be the Mother of Jesus.
One is not the same as the other
When you allow God to be "GOD" then you will be able to comprehend that He will do and accomplish things that go beyond our own human comprehension.
Of course, Mary could have said no in which case God would have chosen an alternate.
HOWEVER, Mary found favor with God because He knew her heart and knew that she would be willing and obedient to accept the opportunity of being the mother of the Saviour of mankind.
In other words, God chose the best candidate for the job and, being all-knowing, He knew that Mary would accept, albeit she was totally surprised and puzzled at first about how she would be able to bare a child when she had not "known" any man.
A "human example" is this: The richest corporation is on an international hunt for the best candidate to introduce the next product that will revolutionize the world. This product will one day bring world peace, end evil and allow users to live eternally.
The candidate must be completely honest and be an individual whose moral and ethical standards are above reproach.
After a computer check of all qualified individuals one name comes up as to who is the best candidate. That candidate is
Nucklesack.
A company representative is then sent to meet Nucklesack and delivers the following message:
Mr. Nucklesack, you have been selected to engineer the next revolutionary invention into this world. Our company has conducted a world-wide search and found that you are the best candidate to accomplish this. What you will produce will bring peace into the lives of individuals and give them eternal life. What's more, this will be given Free to anyone who asks. This will revolutionize the world and people will call you highly favored for bringing this into existence.
Nucklesack replies: "But, I don't have engineering background nor do I have a degree."
Company representative replies: "Not to worry, Nucklesack, you have been found to be the most trustworthy individual in the world. You don't need any engineering knowledge nor an engineering degree. Our company will develop everything and all you need to do is be the one to introduce it to the public. You will be the link that makes this happen in the world."
Will Nucklesack say "No" ?