I've been freelancing for over 15 years; occasionally, a client asks me for help putting together a team of freelancers for a large project, so I've had the opportunity to review many test pieces/writer applications. It's possible the OP submitted a flawless sample and got rejected anyway, but I can't tell you the number of times I've received nasty emails from people who submitted error-riddled sample pieces and insisted that what they had written was perfect. The reality is that many people submit samples with multiple errors and/or don't follow the instructions when completing a sample piece.

I'd say that following instructions is even more important than flawless writing, especially at companies that have editors on hand to review each order. If someone submits an order with three typos in it, an editor can quickly fix the typos. When writers don't follow instructions (don't answer the prompt directly, don't follow the required academic citation style, exceed the max. word count or fail to meet the minimum word count, etc.), it takes a lot more time to fix up the order and make sure it meets the client's needs.

It's honestly a shame that so many people have resorted to scams and dishonesty because there are many intelligent people in Kenya who have a good enough grasp on English to write for American or UK/Australian clients. The company I write for has no restrictions on country of origin, so if a Kenyan writer passes the initial test and does well enough on a phone interview, s/he's more than welcome to write for us. In the time I've been with the company, exactly one Kenyan writer has been able to maintain a consistent level of quality without plagiarizing anything. About 98% of them are fired for plagiarism within a few weeks (or days, for the ones who aren't crafty enough to at least spin their plagiarized content to avoid detection); the rest go downhill immediately, which leads me to believe they aren't the ones writing their sample pieces. Some applicants have gone so far as to steal U.S. Social Security numbers, apply under other people's names, and pay Americans to do their phone interviews for them to conceal the fact that they're not who they say they are.

It's too bad that the people who would excel at this kind of work are missing out on opportunities because some bad apples have cast the whole barrel in a negative light.