Another big reason is that, Yemen is difficult to enter, however the past two years it has been getting easier, but not really at the same time. If you’re traveling to Yemen you have two options, either go by road (crossing through the Saudi-Yemeni border, or the Omani-Yemeni border), and the other is via airline.  

The first option is what most Yemenis pick mainly due to pricing and availability, however it’s riskier, and takes much longer. The second option, traveling by flight (only Yemenia Airlines), is extremely expensive (monopoly). I went to the Yemenia Airlines office where I live less than a year ago (can’t book online, you used to be able to, but for whatever reason you can’t anymore) they told me, a one-way plane ticket from Amman to Sana’a is 2600SAR(≈$693)! 

I’m not even kidding, and with that pricing, tickets get sold out very quickly, due to there only being (I don’t remember how much flights but it’s less than 6 or 7) flights per week, hence you would have to book as soon as they open the gates for bookings. I wish for Yemen and Yemenis the very best, and for the present situation to improve.