Why is buffer capacity at a maximum when the ratio of its components is 1?
With a buffer, you're trying to maximize your ability to neutralize acid or base. The log dependence of the pH on the ratio of interest in the Henderson-Hasselbalch equation is why the buffer works well:

$$\mathrm{pH} = \mathrm{pK}_{a} + \log\frac{\ce{[A-]}}{\ce{[HA]}}$$

Now, your ability to neutralize acid is determined by the amount of the conjugate base $\ce{A-}$. Your ability to neutralize base is determined by the amount of the acid $\ce{HA}$. When the ratio between these is 1, you have equal ability to neutralize either. If you're skewed too far one way or the other, then you will not have much of the respective species around to neutralize added acid or base.