Why is that the case?
Because most ex-Calvinists becoming Lutherans had the sad experience of being burnt by Calvinists teachings. You can read about their testimonies in the Internet. However, in their excitement to hurriedly know Lutheranism, rather than scrutinising the Book of Concord and going through the Scriptures evaluating if the Confession complies with Scripture, they are coached instead to read C F Dubya's works. The light starts to burn specially bright when they get to read how C F Dubya hated Calvinism.
Calvinism is not just confined to TULIP, but it is a whole world view, a paradigm, a spirit of doing theology. It is no mystery but it is well documented in history books that Lutherans have had issues with Calvinists as well as Zwinglians alike.
Where I came from, we have a funny saying, it is not the cops who hate thieves, but fellow thieves themselves.
Of course, when Lutherans read of Walther's critique of Calvinism, the Lutheran hears him singing their refrain. Walther did not like Calvinism, but his paradigm in collapsing atonement with justification and in his belief in unconditional election, makes him a psychological Calvinist himself. What is a psychological Calvinist? It is someone who operates in Calvinistic paradigm without realising or being conscious of it.
Unfortunately, only a few well educated and astute American Lutherans can see and recognise this in Walther. Those that protests and who point this out are ganged up, bullied and hurriedly shut up.
My sincere advice for former Calvinists or anyone from another denomination studying Lutheran theology, do not go to the works of the Lutheran Fathers first, rather go to the Book of Concord. Read it with your Bible open, read it critically and match what it says with Scripture because if you don't you will be floating in the air, still a Christian, but without a confession.