Romney's Mormon Church fiercely attacked California's LGBT community with support of Prop 8.

[Bluenorthwest]

21. First, Kerry and many other Catholic politicians regularly vote in ways

which are counter to the teachings of that Church. Kerry is pro-choice, RCC is anti-choice for example. Kerry and others have been publicly criticized by other Catholics, Bishops, Arch and otherwise, media spokes people, you name it. Here is a link to an article about some Bishops and Archbishops threatening Kerry with a denial of communion for his political and social actions. http://www.cbsnews.com/2100-250_162-610547.html
Mitt IS a Bishop of his Church. Kerry is chided by Bishops of his Church. And here is a piece showing that it was Ratzinger now Pope who called for Kerry and others to get the boot.
http://www.talkleft.com/story/2005/04/20/684/83575

Secondly, John Kerry and generations of American Catholic politicians have spoken clearly about their faith, it's relationship to their work in politics, and said openly that these are separate things. The first Catholic to contend for President, JFK, gave a famous and historic speech regarding his Church which left no qualm unanswered. Obama had to do the same because his Church was not white, I guess, but he certainly did do a 'my faith and my work' speech. It was, like Kennedy's a really great speech. Like Kennedy, that speech helped get him elected.
With Bishop Romney, we get the idea that his faith and it's relationship to his secular work is the last thing he wants to talk about- and frankly, most of us have met many Mormons who always come to the door and nearly beg to speak of their faith. Bishop Romney is the fist Mormon I have encountered who avoids speaking of his faith. Thus his reticence seems intentional and self serving.