Because most Catholics don't know about it.
Typically a person who argues against Catholic doctrine will say something like this: Jesus didn't condemn homosexuality (to use one example) he never talked about it; the epistles don't really refer to homosexuality but other sexual practices.
Of course, you can do the proof-texting thing and show that yes, the New Testament did condemn homosexuality but then they always say "but that's not what the verse really says." And then they put their own spin on it.
And that's why Sacred Tradition is necessary: to bring an authoritative voice to doctrinal disputes.
Sacred Tradition is nothing more than a universal doctrinal consensus that can be traced back to apostolic teaching. It is a mode of transmission of Revelation that is equivalent to Scripture.
It only makes sense that Scripture and Tradition are equivalent. Both are of apostolic origin, both testify to what the apostles believed, both are guided by the Holy Spirit-- Scripture through divine authorship, Tradition through Church infallibility.
Sacred Tradition is expressed through an unbroken witness of Church Fathers.
So when non-Catholics argue that the Bible does not condemn homosexuality, first-- they're wrong-- but secondly, Sacred Tradition confirms the Church in her understanding of what the Bible says. Texts don't interpret themselves, and textual interpretation can be subjective. That's why we can argue over meaning, but you can't argue against historical consensus. When every mention of homosexuality by Church Fathers from apostolic times is accompanied with condemnation, there can be no doubt about what Divine Revelation had to say about sodomy.
And that's the usefulness of Sacred Tradition. Every Catholic should be taught this, and every one who wants to argue Church doctrine should know about it.
Link Dump
New Advent has a Cardinal Buzz Meter, that uses Google Trends to measure who is being talked about.
The CBC and Radio-Canada will each have an interview with Cardinal Marc Ouellet. Both stations will feature him on Monday and Tuesday.
Xt3 has a conclave blog, but I haven't been able to find an RSS feed for it. And by the way, I've been signing in to XT3 more often lately. I'm starting to get the hang of the place. It's a lot like facebook in its first generation.
Sudan has amputated a man for armed robbery.
