I've no idea why it was changed. There were a handful of edits Lewis made for the US editions that some have argued are improvements, but most of them don't make a real difference, and this one is by far the silliest. Since when does a lion's roar sound like "Wow!" — even from a lion in our world, let alone from THE Great Lion??

And it doesn't help that apart from not being good onomatopoeia (I love that word), "Wow!" is normally used (at least by human beings) as a cry of excitement or amazement, not as a  noise you'd make to frighten off a Witch. I remember one Narnia commentary I read suggested that the word "wow" was more in use in the US than in Britain at the time Lewis was writing and he wasn't aware that it meant something, but I find that a bit hard to believe.

It's worth noting, though, that there are no "US editions" in modern printings of the Chronicles — for about the past 30 years (I think this started in 1994, but I can't remember where I found that info), all English-language editions worldwide, including in the US, have had the original British text throughout. I've never quite understood why there were different American editions in the first place. 

In Lewis's defense (I'm assuming he made this change since he also made the Fenris Ulf change for the American edition), "Haa-a-arrh" is really hard to say when you're reading the book out loud. But, yeah, "Wow" isn't what I'd call an improvement.  

It's a pretty funny image, Aslan suddenly barking out 'wow!' like that. I don't see how that works at all. Maybe more of a 'mrowl!' like a warning cat noise, but more booming, would be the closest to that which could work.

Not really a fan of the original wording in books to be changed. Though I can understand in certain circumstances it may be necessary. I don't know how much British books are edited in the US versions now? Certainly the US-to-UK system now seems more often than not to retain all the US terms and grammar differences. (Any further thoughts on that I won't get into here.)