Why is atque preferred before a vowel? Isn't the -e just asking to be elided there (in ordinary speech, not just poetry)? Why not just go with ac?
The shorter ac is not the same thing as atque with elision. The former is a short (light) syllable, the latter a long (heavy) one. Latin seems to want to preserve syllable length in sound changes, so its importance is not limited to poetry. So yes, the -e will be elided, but there still is a difference between atqu' and ac.
The original form is atque (at+que), and when words are changed, Latin has a tendency to preserve syllable length. There is nothing wrong with ac per se, but there is something wrong with turning atqu' into ac, because such a change would turn a long syllable into a short one. The reason is historical; it has to do with where the word comes from and how sounds can change in Latin.
For a concrete example, take Vergil's Aeneid I.464:
Sic ait atque animum pictura pascit inani
The fourth syllable of the verse — the first of atque — is long because it is followed by a consonant cluster. If you were to replace it with ac, the syllable count would be the same, but the fourth syllable would become short, invalidating the hexameter. The single syllable of ac is short when followed by a vowel, and this is precisely the setting you asked about. I chose an example in poetry so that the effect of syllable length is emphasized, although the effect is not limited to verse.
Of course this would be circumvented by pronouncing ac as acc, but that would probably be more confusing than using atque with elision, which makes pronunciation self-evident. Remember that elision is not specific to verse, but is also a feature of prose in classical Latin.
This doesn't explain why or how atque is shortened to ac before a consonant, but that wasn't your question.
This question has been asked and answered before:
To save you time, Allen's book on the subject is online at a url in the second of the answers given there.