I whole heartedly believe it is because there are not enough of us Somali males to go around. What do you expect, that these girls stay single all of their lives? I guess that is why they have to settle for marrying and dating outsiders.
Another thing is that many Somalis are overly suspicious, and I find that young men are increasingly suspicious nowadays for whatever reason. Many have grown accustomed to the easy single life, watching football on their flatscreen TV, cooking and eating takeaway meals. Can't be bothered with responsibilities and western women which seem to come of as a headache more and more to young men.
But if you want to marry a decent sister, I believe it is quite easy, just depends on how you go about doing it. I know plenty of girls who are desperate to get married to a Somali man, but cant find any.