On the specific question, no I don't know why. As you can see in @AnkitSharma's answer, one answer was converted to a comment by me, but it was not protected at that time. 2 days later the comment was deleted and the question protected.
On the general point. IMHO there is probably no good reason to protect a question like this. Its more typical to react to a question that has attracted 2-3 poor answers from low rep (usually unregistered) users.
However being proactive can sometimes be helpful. Sometimes you can 'just tell' when a question is likely to attract multiple poor answers - for example its a very hot topic, the question is getting a lot of views quickly, its a question that might be somewhat controversial and/or has already got one poor answer. For example TylerShads protected a Game of Thrones Question about this Sunday's episode, that already has 13k views and one poor answer.
An example of a more reactive stance is this question about Top Gear which is a year old but has 10k views and has a result has attracted 3 very poor (now deleted) answers - that are basically 'forum-like' comments. I protected this yesterday, as its clearly got a history and with the general popularity of Top Gear is likely to continue to attract answers like this, and eventually be protected by Community anyway.
I would stress however that moderation is a judgement call - mistakes are made. High rep users here can unprotect if they think its unnecessary, or if the time in which it might have been necessary has passed. Ping a mod in chat if you are unsure, or don't have the unprotect powers yourself. I personally have a 'lighter touch' than TylerShads - but put another way that means I'm being less proactive about protection.