Why characters with "口字旁" were chosen in this transliteration?
Characters with "mouth radicals" are often chosen for transliterations. This is especially helpful to show that it is a proper noun rather than anything else, otherwise the reader might try and put meaning into characters that are simply there for their associated sounds.

A similar concept can be seen in Chinese onomatopoeia. For instance "choo-choo" the sound a steam train makes can be written: 呜呜 in Chinese, and again we see the mouth radical to indicate purely sound-based usage. 咩咩 is equivalent to the English "baa baa," the sounds that sheep make. A closer fit to English would be: 喵喵 the sound cats make. 

As a note: this is quite an old transliteration practice. You wouldn't see places written this way today and if you look at most countries names they just simply are not translated like this. But, the mouth radical does hint at sound over meaning.