For everyone except evangelical Christians, the answer is obvious. It's because they hate gay people. For evangelical Christians that doesn't explain anything, because they are quite certain that they don't hate anyone, and certainly not gay people. They really are trying to help.
This is a book by a conservative evangelical for conservative evangelicals. For those outside its target readership, the response may well be (as Andrew Brown has put it in a Guardian "Comment is Free" piece called "Heartbreaking progress"):
"an American book suggesting that gay people have reason to distrust the church, and perhaps the most important thing about them is that they are people, rather than that they are gay. Well, duh."
For those within it, the effect has been almost miraculous, with Andrew Goddard describing it on the Fulcrum website as:
"...a book like no other I know, a book which desperately needed to be written, a book which sadly very few people could write, a book which every Christian - or certainly every evangelical - who wants to learn about homosexuality and a Christian response to gay and lesbian people - should read.... The book is, however, really not ultimately about homosexuality. It is at heart about mission and in particular about what it means to be Christ-like towards a community which Christ's followers have hurt and alienated, towards people we think we can tell the truth about and to but whom we basically do not understand, and towards an important cultural sub-group who have become in many ways our social and political enemies and we are therefore especially called to love."
The fact is that this book seems to be a magic bullet which might, might, change fanatics into human beings, remembering Winston Churchill's definition of a fanatic as "someone who can't change his mind and won't change the subject." If it is, it will be doing the church an even greater favour than it does to LGBT people who come into contact with the church.