Blizzard didn’t elaborate on this beyond saying the bird spirit guided them there and they eventually forged a pact with the Greens some time before WOTA. The problem you run into is that the events leading up to the conception of the centaur were extremely specific, on top of needing one of the most powerful earth elementals and a keeper.
Until Blizzard manages to come up with a satisfactory explanation it’s in the “They ed up” category.
I bet Erevien doesn’t like the new centaurs simply because they and their society feel more like “Alliance centaurs” (like, more Humans than horses ) so just for this reason the old and savage ones are better (even if most of them are hostile to the Horde).
Also the original idea for Garrosh going evil, it was about him creating the “Mongrel Horde” in which all savage races like Gnolls, Centaurs and even Murlocs were going to join and fight for him, and we had to stop them. Then the devs changed their plans and made him an Orc supremacist instead with the True and Iron Horde, but originally all the centaurs were going to fight for Garrosh so probably he likes them for that, too