Why was this question protected? It says that it has attacted low quality answers, but the top answer has over a 150 votes and and second place has nearly 100. Both answers are detailed. There are only 6 answers, and IMO only the bottom 2 come close to being low quality (neither would have been stopped by protecting the question).
How many low quality answers are sufficient justification to protect a question?
There's no hard limit -- it's solely based on moderator's discretion. We tend to protect very popular questions that are somewhat "soft", that is, many people might have similar (but not exactly the same) observations and opinions, which don't really contribute much additional information that hasn't been mentioned in another post. Popular questions attract a lot of new users, which might be under the impression that SE sites are similar to traditional forums, where 20 pages of posts is not unusual. SE network is different, so in the interest of preserving high signal-to-noise ratio, some form of throttling new answers is desirable -- hence the protection mechanism.
Concerning where the threshold is set: for me personally, more than 5 or 6 answers to a question (even if all are good) is typically a warning sign that protection should be considered. This is not a hard limit by any means, just some practical limit that might indicate a question is in trouble. Protecting is also much more likely if there are deleted or converted to comment posts from new users.
The reputation threshold to post is so low, that for any user who has already contributed to the site should have enough to answer a protected question. By all means, if you have a genuinely new observation, opinion or a reference different from the other posts, feel free to answer a protected question; if it's mostly the same, it might be better to add a comment to a relevant post.
Questions are protected based on things you don't see, almost by definition. Spam and really poor (non-)answers get deleted and the rest is generally OK. Usually, they are also questions that attract a lot of attention so they will also have good answers, lots of votes, etc.
It does not prevent the question from staying there, (most) people from answering it, answers to be upvoted, etc. and is completely unrelated to the quality of the question or answers that are still visible. It just prevents unknown users from posting junk that has to be deleted.
I am not a moderator and have no particular insight in their decision process but I can see deleted answers and, in practice, one bad answer on a popular question seems to be enough to justify a protection (that's bad as in spammy or gibberish to the point of being deleted, not bad as in debatable or low in quality).
