The Mormon Church is attacked by quoting from Journal of Discourses, Mormon Doctrine, Book of Mormon, Comprehensive History of the Church, or quotes from various other diaries and sources. Then we are challenged to explain that quote or statement. The quote is presented because it gives the required impression even though that may not be a correct interpretation when the context is fully understood. The Christian critics have no accepted body of beliefs or reference works that are considered universally binding or accurate. The only standard that is universally accepted is the Bible, but even then you may have to consult different versions to find acceptance. My experience is that these Christians don't allow others to suggest a correct interpretation of the scriptures. When challenged about something another Christian says, they don't feel obligated to agree with those statements or interpretations.
This is in contrast to the doctines and beliefs of the Church of Jesus Christ of Latter-day Saints. The Mormon people have additional scriptures, and authoritative statements from prophets that establish a complete structure of belief and practice. All the members of the church are required to believe that which is true and a significant body of literature has been written to establish the various points of doctrine. Since the church has a standard that we all believe, it provides a large target for critics to attack.
However, probably the main reason critics have so much fun attacking the church is that the Mormon beliefs do not conform to the norms of other Christian ideas. Some concepts such as polygamy seem especially strange when they are first examined. Some Mormon beliefs seem easy to discredit, but on deeper examination it is found that there is more to be considered.
Also the Mormon people are arrogant enough to proclaim that they are the "only true church." When a person leaves the church they cannot easily join another church. They have been taught and believed that other churches were wrong. Without belonging to the "true" church they are sometimes left with no church. They might join the ranks of those who are cynical and skeptical about religions in general. Another possible path might lead to acceptance of membership in another religious faith, and most likely they will retain feelings of betrayal and possibly hatred toward the Mormon church. With those feelings of apostasy they might become very active in "winning" others away from the Church of Jesus Christ of Latter-day Saints. If they are successful in causing others to doubt their testimony of the Mormon church, they feel justified in their own decision to leave.
Another reason that the Mormons are an "easy" target is that they actually claim to have living prophets and apostles. Our pre-conceived ideas about what a prophet is usually does not include anyone that actually walks and talks like other men. In fact even members of the church cause some problems by not allowing Joseph Smith to be a man as well as a prophet. We have the opportunity to get a close look at what a prophet really is and not what we might imagine a prophet should be.