I'll concede that Elminster has mostly faded from the picture nowadays. But he looms large enough in the history of FR that I can't see how he could be left out of a set like this one--I mean, if you're gonna reach back to Minsc and Boo, and make alt frames that echo the old BD&D covers, that's well into the Horny Gandalf era of the Realms.
I'm still surprised they didn't tap him to be one of the planeswalkers. I'll be utterly shocked if he isn't in the set somewhere.
It's a possibility, though certainly a more negative interpretation. Some other speculation on my part;
1. They're confident this won't be the only D&D themed set and want to save an iconic figure for the next one.
2. We're too close to Strixhaven to give Horny Dumbledore a card.
3. They asked Greenwood and he didn't want Eliminister in the set.
Regardless, if he doesn't make it in the set it will be a common question as to why, so I bet we'll get some kind of answer. It may be the generic "D&D has so many iconic characters we couldn't possibly include them all" though.