To my mind, centaurs didn't appear in the 4E MM and aren't in many adventures precisely because they fall into the awkward "not really a monster (in the evil/killing them for their loot is fine for good parties) but not a basic PC race/likely race for henchfolk" space. In some ways, its similar to blink dogs, except that blink dogs are more clearly good. You can totally have centaurs that the PCs fight. You can also totally have fun, interesting interactions with centaurs that PCs don't fight. But 4E's MM was totally "the book of things to kill" and D&D centaurs are a little awkward for presenting as "just another monster to kill."
I think that philosophy was misguided, and I checked the box for centaurs (one of the few boxes I checked--I'd like to see most of those critters in a MM eventually, but there are only a handful that I'd really feel disappointed that they were missing if they're not in the first monster book/boxed set/whatever.) But I can understand why they would be viewed as more marginal than, say, orcs or dragons or beholders or mindflayers or ogres or displacer beasts or other iconic to the point of "any reasonable initial D&D monster book needs to include these." For me, centaurs fall into the category of "should be included in the initial D&D monster book, but many reasonable versions of it wouldn't include them."
And hopefully, if lots of us checked the centaur box, the whole issue will be moot--WotC will see that lots of people want centaurs and will include them.
As an aside, there's certainly room for a more mythologically grounded version of centaurs who are primarily raiders and bandits and rampaging hordes who could be treated as straight up monsters--an "often chaotic evil" alignment sort of category--but that's not typically how D&D has presented them. And because that's not how D&D has typically used them, I would prefer for the default centaurs to be the neutralish horse people, with the rampaging half-horse, half-human centaurs presented as "some tribes of centaurs skew towards evil and chaos" instead of giving up D&Dness to get more mythological.