Certainly a very personal opinion but the first time I tried Anchor beers sometime in the noughties I was underwhelmed. Don't get me wrong: they're certainly better than Bud, Coors, etc., but as a Brit coming from somewhere with a by then thriving real ale scene, Anchor's beers just weren't that great. More than that, they weren't up to the standards of many other US-brewed craft beers I'd tried, many of which are at least as good as anything the UK or mainland Europe has to offer[0].
The microbrewing scene in the US has really taken off over the past 20 years so, to me, it felt like Anchor just got left behind by it. And maybe Stone Brewing have a bit more to offer[1] and it was easier for Sapporo to buy them and roll with it than to try and revitalise a brand seen as ailing? I'm not saying I like it but I can certainly imagine that perspective within the company.
[0] By the mid-2010s there did seem to be a tendency to overegg IPAs by making many of them ludicrously strong, and just trying to cram in too much flavour, not to mention the consistency of many was borderline gloopy (more like drinking blood than beer) - mercifully I think this trend may be reversing.
[1] Pure speculation: I don't remember ever trying any of Stone's beers so have no comment to make on their quality or otherwise.