Well, I am no expert on such issues, but I do know that Sinai is quite a large area, and parts of it are a hornet's nest with various groups of terrorist (at this point, I think we can assume it is mostly Islamic-based terror organizations? Such as ISIS, Islamic Jihad, Muslim Brotherhood and similar guys).
Then, these guys (the terrorists) have probably got a good assortment of weaponry arriving from Sudan, Yemen, Libia- it doesn't matter if these countries are the source of the weapons or just a proxy through which it is shipped forward from other countries to the terrorists.
Last, there are probably several ways to down a plane that doesn't involve a long range/long altitude surface to air missile. For example, who knows if someone in the ground crew is not a terrorist supporter? I guess the airport screens all the passengers and their luggage- not sure they do the same to all the maintenance, cleaning, fueling, catering etc. etc. there are practically dozens of people with access to the planes. Who knows one of them didn't become an extremist supporter, and helped place a little surprise? A small package placed in the right place can do a lot of damage to an airplane at high altitude (and lots of fuel too).
Problem is, that the same applies for most locals working at the tourist industry. I am only guessing that the police is going to crack down hard on many of the bedouins that work in the tourism industry in Sharm and vicinity.
VERY Sad news.
Egypt is an amazing country- hope they overcome all the difficulties.
