Why is it so difficult?
I see a lively discussion about pros and cons of native and non-native speaking teachers, but the reason why it's hard for non-natives may have little to do with whether or not native speakers really make better teachers. I suspect employers tend to be unwilling or unable to verify the abilities of each job candidate for themselves, so they use "Native Speakers Only" to narrow down the pool of candidates. It may seem lazy, it may be unfair, it can be racist, and it may prevent them from hiring the best possible candidate in some cases. But, hey, it's easy, it's safe, and it's not their job to help those people who may be non-native speakers but can do a better job; they are employers, not career counselors, after all. This happens not only in ESL/EFL but also language teaching in general, writing/editing, and translation/interpretation. I'm not defending them; I just doubt it's a scientific issue of relative merits of native vs. non-native speakers.