Because (1) you can do everything Rome tells you to do and still perish, and (2) the religion of Rome makes no promise to be the unique way to heaven. (link) Now, obviously, these are not the traditional doctrines (pre-Vatican II), but I don't think I could be a Sedavacantist for other reasons.
Warning: the priest in the video uses some ironic bad language in establishing point (1), and actually goes so far as to claim that atheists and Hindus will probably get to faster than him (an idea that I've never seen dogmatically defined anywhere in the documents published by the Vatican).
Second Warning: undoubtedly there are going to be folks who call themselves "Catholics" who disagree with this priest. Of course, 99% of them are not priests themselves. Regardless, even if someone will say that this priest just misrepresents "Catholicism," then the question is how those who make that claim feel qualified to make it?
I think the answer to that question is intuitive: that we exercise judgment (personal judgment) and appeal to a higher authority. Eventually, though, appeals to higher authority run out. Where's the stopping point? For the Reformed believer, the answer is - essentially - Scriptures. We accept the Scriptures on faith, and they are the authority by which we determine whether our churches are right. We do that, because Scripture is of greater authority than men - even than the men of the church. We could accept our churches on faith, but then we'd have no protection in the event that our church apostatized.
Since God has not promised to keep individual churches (even the church at Rome) from apostasy, it seems more reasonable to put our faith only in that which is deserving of it, namely the Word of God contained in the Scriptures of the Old and New Testaments: because by trusting the Bible we are trusting God who communicated clearly in it.