Precious Roy, you are wrong. Really there are not appearances. In 2012, Wendimu was number 11 in 2:07, and went home with a t-shirt...
The fact is Dubai is the fastest course in the World. Perfect tarmac, no turns, normally very good weather around 15 C, very even during the competition, and, being at the beginning of the season, considered the best option by Ethiopian runners, since Ethiopian Federation normally choses the athletes for the Championships (or Olympics) looking at the best 3 times, and, if in shape, there is no place where it's possible to run so fast.
The lack of appearance can also explain why Dubai is the most important Marathon for beginners, who can't have appearances in other Marathon.
An athlete running 2:06 can also win a small prize (in 2012, Seboka Tola was n. 9 with 2:06:17 and won 5000 USD), but at the end has a time allowing him to be invited in the most part of top Marathon (Majors apart).
In 2012, 2013 and 2014, how you can see in the presentation of the race in LR, the winners were at their first Marathon, and practically Hayle Lemi, last year, was in the same situation, having run one small Marathon only before Dubai.