Aside from there potentially being more answers than questions, as well as people more likely to vote on answers, answers seem to be easier.
Case in point, extremely well-read questions like this have caused a single Great Question Badge, but a multitude (22) of Great Answer Badges. This is of course an extreme example, but I think the point stands.
But I think it's about what you're posting about - if you're writing a question, you need to establish all your context aside from the subject of the site, while when writing an answer it's in the already established context of the question.
In my opinion, that makes writing a good answer easy, because:
- For an answer, you can often just address the question point by point.
- For a question, you need to come up with all important things instead, as well as related side-concepts.
Of course, the best answers often also reference related side concepts. But I think it's easier.