(Well, you're a very astute observer. To be fair, though, to Mormons):
#1 Mormonism is very difficult to defend on the level you mention.
#2 Usually at least half of those "mockers" you've observed aren't Lds at all. As to why they mock, can't pretend to speak for all or even most of 'em.
I know some have revealed they have family ties (or friends) who are Lds;
others just like to see various perceived conservative sub-groups within Lds "get along" vs. anything that might "split them"...seemingly totally unaware, or blissfully ignorant on purpose of the wholesale Mormonesque accusations their church has plastered upon Christianity going back almost 200 years now.
To them, they think we have "bigger battles" to fight -- and they just don't seem to know how others could possibly multi-task such struggles on various stages.
Overall, I also take "cultural factors" into consideration: Frankly even many conservative FReepers have been "schooled" by beloved high school educators & college profs in the secular "tolerance" worldview. So...they think we ought to tolerate (almost) all the isms out there ... ...the "exceptions" that come to mind are ones like Radical Islamism; certain brands of liberalism; communism; socialism, etc.).
And what's always of interest to me is how these individuals tend to preach tolerance of others' various religious expressions -- all as they disdain my religious expressions. (Makes for some mighty interesting self-refuting, hypocritical testimony on display here! :))
Major kudos there. Thanks.