articles - What is the articulatory logic behind the "a/an" rule in English?

Is there some articulatory reason behind why we choose to preface consonant sounds with the article a and vowel sounds with an? The reasoning I've read in the comments somewhere, I don't remember exactly where, is that we are trying to avoid consecutive vowel sounds, but why exactly are we trying to avoid these consecutive vowels? Are they harder to articulate for some reason? (If so, can that be explained as precisely as possible?) Are there many cases that can pointed to in English where we similarly avoid consecutive vowel sounds? Why not dipthongnize the vowels? Is the whole thing mere idiosyncrasy on English's part?