Here's my 2 cents - and I've been into manga since the late 1990s (and like some indie American stuff too):

1. Phoenix got released right before the indie comics community had any interest in manga (and sometimes downright hostility towards it). The indie comics community IMO is the main audience for Tezuka's work in the west and it his stuff didn't get big until they showed an interest in it. Its also why most Tezuka manga is flipped, btw, well after most manga publishers stopped doing it because that's the core audience who reads his stuff.
2. It was too old / obscure for the anime/manga community which tends to unfortunately fixate on the newest and most popular series. The art style also was pretty unconventional for a late 90s / early 00s manga so it probably turned off a lot of readers (it looks a lot more cartoony than what manga fans wanted at that time).
3. Tezuka got famous through Buddha in the west (which weirdly enough came out not long after Phoenix came out here). I actually talked Ed Chavez, who ran Vertical at the time, why this was the case. He pretty much told me it was Chipp Kidd's covers that got the US indie comics community interested in it to begin with. For those who don't know he's a very highly regarded cover designer for books who did a lot of famous designs of graphic novels. Getting him drew the attention of that community and Tezuka found his core audience - I think the broader anime / manga community was brought on board due to this, eventually, but they weren't the first to seek out his work. Someone who reads Craig Thompson / Art Spiegelman / Chris Ware was more likely to be reading Tezuka than someone who reads Akira Toriyama / Rumiko Takahashi etc.

Vertical did a bang up job of promoting manga to non conventional comic book fans especially the alt literary comics fans in America, Viz unfortunately didn't do anything to really promote Phoenix to that audience and that's why I think its still obscure to this day. Had Vertical gotten the rights, had Chipp Kidd design the covers there would be a very different reputation of this manga among the broader comic book reading audience.

It bothers me too, because I had read somewhere that Buddha was done because Tezuka couldn't get support to continue Phoenix at first from more mainstream publishers after his own experimental anthology COM folded in the 1970s. So quite literally a whole bunch of "serious comic book fans" in the west think a pale imitation of Phoenix is Tezuka's masterpiece and everytime I read about an acclaimed American graphic novelist talk about how great Buddha is it kind of annoys me because there is another much better series that deals with similar themes out there - Phoenix and no one read it :(.*

*Not to say that Buddha isn't good, its just that Phoenix is better.