As stated in Idea behind "Your vote is now locked in unless this answer is edited", the motivation behind locking ones vote stems from an attempt to perturb possible "tactical down-voting".
Tactical down-voting suggests that users could vote down other competing answers in an attempt to possibly let their own answer be more visible (it is possible to sort answers based on votes... in fact, it's sorted by score). While the action is reversible (via an edit to the answer), such a hindrance may be enough of a nuisance to discourage it. And again, while this is easy to do for those with the edit privilege (2K+ reputation), perhaps it would be more likely for lower-rep users to resort to such antics.
Either way, this is a decision made to pro-actively avoid this questionable behaviour and has been rolled out to all posts (not just answers, but questions as well).